Step 1

In a skillet over medium-high heat, add 2 tsp. of vegetable oil. When oil is hot, add the onions and 1/2 tsp. of salt and cook until dark brown in color and softened. Halfway through, you’ll want to turn down the heat to medium heat. Caramelizing onions will take about 20-30 minutes. If you notice that your pan is getting slightly dry, adding a tablespoon of water will help the caramelizing along. The water should sizzle and quickly evaporate. You’re not trying to have the water stay in the bottom of the skillet while cooking the onions.

Step 2

When the onions are caramelized, remove from skillet and place in a bowl. Add the remaining 1 tsp. of vegetable oil to the skillet and add the mushrooms and remaining 1/2 tsp. of salt. Cook the mushrooms until softened, about 5 minutes. Remove from skillet; keeping the liquid that has accumulated in the skillet.

Step 3

Preheat broiler in oven.

Step 4

Take the room temperature butter and butter both sides of bread. Place bread on to a baking sheet and place under broiler for about 1 minute each side. Carefully remove bread from baking sheet and on an even surface, begin to assemble your sandwich.

Step 5

Place caramelized onions on one slice of bread, then the mushrooms on top, then 1/4 cup of shredded Swiss cheese. Gently place on baking sheet. Repeat with another slice of bread. Place baking sheet into the oven and broil for 1-2 minutes until cheese is melted and bubbly.

Step 6

Remove from oven and place the remaining two slices of bread on top of each and gently press down.

Step 7

Slice sandwiches in half and serve!