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Portobello Croque Monsieur

Active Time: 30 minutes
Total Time: 30 minutes
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Why we love it:

I love vegetables but I do eat meat a few times each week. The one thing I don’t have any need for is pretending like vegetables are “meat substitutes.” A Croque Monsieur is a ham sandwich, but this one is made with mushrooms. I would never try to say they are “ham-like” or it tastes the same because they don’t! They are mushrooms and they are so delicious combined with the cheese and bechamel in this preparation. Consider frying an egg for the top and call it a Croque Madame!


  • 3 Portobello Mushrooms (or 4, about 1.5 pound)
  • 1 Tablespoon Extra Virgin Olive Oil
  • 1 teaspoon Balsamic Vinegar
  • 4 Tablespoons Unsalted Butter (plus 2 additional teaspoons of butter)
  • 14 cup Flour (all purpose)
  • 112 cups Whole Milk
  • 1 Tablespoon Whole Grain Mustard
  • 12 teaspoon Nutmeg (freshly grated - use only ¼ teaspoon if it’s pre-ground nutmeg)
  • 12 teaspoon Kosher Salt
  • 2 cups Gruyere (grated, divided)
  • Bread (8 slices white - country style, brioche, thin or thick all work)
  • 4 cups Spring Mix Salad Greens
  • 4 Tablespoons Salad Dressing (vinaigrette, or oil and vinegar)

Cooking Instructions


    Remove the stem from the mushrooms and slice the cap in ½ - not strips, but the other way, so you have 2 thinner rounds. One might have a hole or two in it, but that’s ok.

    Heat the oil (and a dot of butter if you like) in a large sauté pan and sauté the caps until they’re browned and mostly soft, about 5 - 6 minutes, flipping once or twice as needed. When they’re all soft, pour in the balsamic vinegar and let it sizzle for about 30 seconds and turn off the heat. Set aside.


    Melt butter in a medium saucepan over medium heat until foamy. Add flour all at once, while stirring constantly until mixture is pale and foamy, about 3 minutes. Gradually pour in the milk, whisking the whole time, until the mixture is smooth. Keep cooking, and stirring very often, until the sauce is thick, about 4 - 6 minutes longer. Remove from heat and whisk in mustard and nutmeg. Season with the salt.

  • 4. GRUYERE

    Grate the cheese or slice it really thin. Stir ½ of the cheese into the bechamel sauce and whisk until it melts.


    If you can assemble the sandwiches next to the stove so you can keep stirring the bechamel, do that. Otherwise, just wait until the sauce is done for this step.

    Make 4 sandwiches using the rest of the cheese and the mushroom caps. Layer the mushrooms on the bread evenly. Place on a baking sheet. Wipe out the pan you used to cook the mushrooms with a paper towel. And return it to the stove.

  • 6. GRILL

    Use the extra teaspoons of butter in the mushroom pan to make typical grilled cheese sandwiches. Sear one side, then flip when it’s browned and do the other side. You can leave these a little bit lighter than you would if just eating a regular grilled cheese. When done, transfer to the baking sheet.

  • 7. SALAD

    Wash and dry greens as needed. Toss with salad dressing.

  • 8. SMOTHER

    Pour the bechamel over the top of each sandwich in the center and use the back of a spoon to smear it to the edge, it can drip a little bit down the sides. Place sandwiches into the broiler and keep an eye on them for about 3 - 5 minutes, perhaps rotating the pan depending on your oven, until the bechamel is bubbly and just a little browned.

    Remove and let cool for a minute or so. Slice in half.

  • 9. SERVING

    Serve mushroom croque monsieur with salad on the side.

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    I have not tried this, but I see no reason this wouldn’t be delicious with a dairy cashew cheeze sauce over top. I’d look up vegan mac and cheese recipes and use that type of sauce over top
  • VEGAN:
    omit butter as well and sear with coconut oil or olive oil
    any kind of mushroom will taste great but just slice them thinly and sauté. They’ll be more like you’d expect from mushrooms on a burger, but that’s ok!
    thin slices of eggplant would be perfect here. Cook in the same way as the mushrooms to start. Thin rounds of sweet potatoes or butternut squash would look gorgeous and taste great too, try roasting them before making the sandwiches so they’re soft. I bet a thin cauliflower steak would also be successful. Let me know if you try that!
    jarlsberg, swiss, or any preferred flavor of melty cheese
    half and half, cream, and 2% would all work out

Kid Friendly Version

How to Tweak This Recipe for Kids:

  • If you can get your kid to try this, I can’t imagine they wouldn’t like it, but also, prepare them a plain mushroom and cheese (or just grilled cheese) and serve a winning vegetable or fruit on the side. It could be fun to slice the sandwiches into long strips, then coat in bechamel to make “sticks.” Use part cheddar, jack or a cheese they definitely enjoy. Lastly, consider omitting or reducing the mustard, that could be a jarring flavor to new palettes

Prep Ahead & Use It Up

  1. Roast the mushrooms ahead of time, they can be assembled into the sandwich cold or warm
  2. You can make the bechamel cheese sauce a week ahead. It will get really thick when you chill it but whisk over medium-low heat to soften it up and warm it before pouring on the sandwich
  3. If you store the bechamel in the fridge, press a piece of wax paper or plastic wrap over the top so it doesn’t form a skin – bechamel is great for pastas and sauces
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