Introduction

This Melt Sandwich combines sweet caramelized onions, savory sautéed mushrooms, and melted Swiss cheese on crispy sourdough bread. It’s a satisfying vegetarian meal perfect for lunch or a cozy dinner. The balance of flavors and textures makes every bite truly delightful.

A grilled sandwich with two thick slices of golden-brown toasted bread, topped with a small bunch of fresh green parsley. Between the bread slices is a layer of melted white cheese that drips down the sides, covering sautéed brown mushrooms that are soft and glossy with a hint of green herbs sprinkled on top. The sandwich sits on a white plate with a white marbled texture surface beneath, and a fork is partially visible on the left side. photo taken with an iphone --ar 4:5 --v 7

Ingredients

2 large yellow onions, thinly sliced
2 tablespoons olive oil
1 tablespoon butter
1 teaspoon brown sugar
1/2 teaspoon balsamic vinegar
Salt and freshly ground black pepper to taste
1 pound cremini mushrooms, sliced
2 tablespoons olive oil
2 cloves garlic, minced
1/4 cup dry white wine (optional, but recommended)
1 tablespoon fresh thyme leaves
Salt and freshly ground black pepper to taste
4 slices sourdough bread (or your favorite bread)
4 ounces Swiss cheese, thinly sliced
2 tablespoons butter, softened
1 tablespoon Dijon mustard (optional)

Instructions

Step 1: Peel and thinly slice the yellow onions.
Step 2: In a large skillet, heat the olive oil and butter over medium-low heat.
Step 3: Add the sliced onions to the skillet and stir to coat.
Step 4: Cook the onions over very low heat, stirring occasionally, for about 30-45 minutes until deep golden brown and very soft. Reduce heat if they brown too quickly.
Step 5: Stir in the brown sugar and balsamic vinegar. Cook for 2-3 minutes, stirring constantly, until the sugar dissolves and the balsamic is absorbed.
Step 6: Season the caramelized onions with salt and pepper to taste. Remove from skillet and set aside.
Step 7: Clean and slice the cremini mushrooms.
Step 8: In the same skillet, heat olive oil over medium-high heat.
Step 9: Add the mushrooms and cook, stirring occasionally, until browned and softened (8-10 minutes). Cook in batches if needed to avoid overcrowding.
Step 10: Add the minced garlic and thyme leaves. Cook for 1 minute, stirring constantly, until fragrant.
Step 11: Pour in the white wine, scraping up browned bits from the pan. Let the wine reduce until nearly evaporated.
Step 12: Season mushrooms with salt and pepper. Remove from skillet and set aside.
Step 13: Lightly butter one side of each bread slice.
Step 14: Spread Dijon mustard on the unbuttered side of two bread slices, if using.
Step 15: Place bread slices buttered side down in a clean skillet over medium-low heat. Layer half the Swiss cheese on each slice.
Step 16: On one slice, layer half the caramelized onions and sautéed mushrooms. Repeat on the other slice.
Step 17: Cook sandwiches 3-5 minutes per side until cheese melts and bread is golden brown, covering the skillet to help cheese melt faster.
Step 18: Remove sandwiches and place one slice on top of the other. Cut in half and serve immediately.

Tips & Variations

Use your favorite melty cheese like Gruyère or cheddar for a different flavor.
Add fresh spinach or arugula inside for a peppery bite.
If white wine isn’t available, use vegetable broth or omit it entirely.
Cooking onions slowly at low heat ensures deep caramelization without burning.
Try whole grain or rye bread for a heartier sandwich.

Storage

Store any leftover sandwiches wrapped tightly in the refrigerator for up to 2 days. Reheat gently in a skillet over low heat or in a toaster oven to keep the bread crisp and cheese melty. Avoid the microwave if possible to prevent sogginess.

How to Serve

A thick grilled sandwich stands in the center on a white plate against a white marbled texture background. The sandwich has two outer layers of golden-brown toasted bread with a crispy texture, sprinkled lightly with chopped herbs. Between the slices, there is a thick layer of melted creamy cheese that flows out over the edges, mixed with cooked, sliced mushrooms that are glossy and browned. The mushrooms also spread around the base of the sandwich, adding a rich texture and earthy color. On top, a small bunch of fresh green parsley decorates the sandwich. photo taken with an iphone --ar 4:5 --v 7

Serve this delicious recipe with your favorite sides.

FAQs

Can I make the caramelized onions and mushrooms ahead of time?

Yes, both can be prepared a day in advance and refrigerated. Reheat gently before assembling the sandwich.

Is there a vegan version of this sandwich?

Absolutely. Substitute butter with vegan margarine, use a plant-based cheese alternative, and skip the white wine or replace it with vegetable broth to keep it vegan-friendly.