Wash and dry all produce. Adjust rack to upper position and preheat oven to 425 degrees. (TIP: Put your baking sheet in the oven as it heats for an extracrispy crust. Just make sure to oil the sheet first.) Halve and peel shallot, then slice lengthwise into thin strands. Trim mushrooms, then cut into 1/4-inch-thick slices. Cut lemon into wedges.

Heat a large drizzle of oil in a large pan over medium-high heat. Add mushrooms, shallot, and half the Italian seasoning. Cook, tossing, until softened and browned, about 10 minutes. Season with salt and pepper.

Place flatbreads on a lightly oiled baking sheet. Sprinkle evenly with cheese and remaining Italian seasoning. Spread mushroom mixture evenly over top. Season with salt and pepper.

Bake flatbreads in oven until cheese is melted and crust is golden brown, about 7 minutes. Remove from oven and drizzle with truffle oil, to taste. TIP: Start with a few drops and add more from there—you may not use all of the oil.

Toss arugula in a medium bowl with a drizzle of olive oil and a squeeze or two of lemon. Season with salt and pepper.

Cut flatbreads into slices. Serve with salad on the side. TIP: If you like, you can spread a little bit of the salad on top of the flatbreads.