Step 1

Bring truffle butter to room temperature. Wash and dry produce.

Step 2

Trim and thinly slice mushrooms (skip if your mushrooms are pre-sliced!). Halve, peel, and thinly slice onion.

Step 3

Heat a drizzle of olive oil in a large, preferably nonstick, pan over medium-high heat. Add onion and mushrooms; cook, stirring occasionally, until browned and softened, 8-10 minutes. Season with salt and pepper to taste. Turn off heat; transfer to a medium bowl. Wipe out pan.

Step 4

Spread one side of each sourdough bread slice with softened truffle butter; place half the slices on a clean work surface, buttered sides up. Evenly top with cheese, then remaining bread slices, buttered sides down, to form sandwiches. Heat a drizzle of oil in same pan over medium heat. Add sandwiches and cook until bread is golden and cheese has melted slightly, 2-3 minutes per side. (Work in batches if necessary.)

Step 5

Halve on a diagonal and serve.