This Croque Monsieur pairs tender ham and melted Gruyere with a silky béchamel sauce and buttery, golden sourdough for a restaurant-quality French sandwich.
Check what you already have. We'll leave it off your shopping list.
Prices vary by store
*Service fees apply. Instacart® and the Instacart logo are registered trademarks of Maplebear Inc. d/b/a Instacart.
CookOS may earn a commission from Instacart orders.
Prep: Preheat the oven to 400°F. Line a large baking sheet (21x15) with parchment paper.
Make roux: Melt four tablespoons of unsalted butter in a medium to large saucepan over medium heat. Reduce the heat to low, then add flour. Cook while whisking constantly for 2 minutes.
Add milk: Remove from the heat. While whisking constantly, add about a quarter of the milk and whisk until smooth (it will be thick). Continue to whisk while adding the milk a couple of tablespoons at a time, whisking until smooth between each addition. Add the salt, nutmeg, white pepper, and cayenne.
Thicken: Bring to a simmer and continue to whisk for a few minutes until thickened to the consistency of spreadable butter or pudding, not runny at all. Remove from the heat.
Slather Béchamel: Add 8 slices of bread to the prepared baking sheet. Spread half of the béchamel thickly over ALL the slices (like you would butter toast- it will be thick).
Assemble: Top FOUR pieces of the béchamel bread with half of the Gruyere slices, followed by all of the ham (folding as needed to fit). Spread a thick layer of Dijon over the ham, then top with the remaining slices of Gruyere. Close the sandwiches with the remaining slices of bread, béchamel side down.
Pan fry: Heat one tablespoon olive oil in a cast-iron skillet over medium heat. Once hot, add the butter to melt. Working in batches, grill the sandwiches for about 2 minutes per side, until deeply golden brown, pressing down occasionally with a spatula so they grill evenly (the cheese will not be completely melted). Transfer back to the parchment paper-lined baking sheet.
Topping: Spread the remaining béchamel on the top slices of each sandwich. (Gently reheat and re-whisk the béchamel with a splash of milk if it's become too cold/thick to spread.) Add the shredded Gruyere and grated Parmesan over the béchamel.
Bake: Bake for 10-12 minutes, until the cheese is melted, then broil for 2-3 minutes until the top is golden.
Serve immediately with a fork and knife to enjoy every gooey bite (its rich, cheesy goodness is too messy and hot to eat with hands).
Store leftover assembled (uncooked) sandwiches wrapped in plastic wrap for up to 1 day refrigerated. Reheat in a skillet over medium heat until warmed through and cheese melts, about 3–4 minutes per side.
Yes. Prepare the béchamel up to 2 days ahead and refrigerate in an airtight container. Reheat gently over low heat or in the microwave before assembling.
Emmental or Swiss cheese work well as substitutes. Avoid pre-shredded cheese; slice it fresh for better melting.
Use thick-cut sourdough or pan de mie, butter both sides generously, and pan-fry over medium heat until golden before assembling, then finish in a skillet or press.
Omit the ham and add sautéed mushrooms, caramelized onions, or tomato slices to maintain texture and flavor.
Black forest ham adds superior flavor and texture, but quality French ham or Swiss ham are acceptable alternatives.
