Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry produce.

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

Place flatbreads on a lightly oiled baking sheet. Toast on top rack until lightly golden, 3-4 minutes. Remove from oven. (For 4 servings, divide flatbreads between two sheets; toast on top and middle racks.)

Meanwhile, heat a large drizzle of oil in a large pan over medium-high heat. Add mushrooms; cook, stirring occasionally, until lightly browned, 4-5 minutes. TIP: Lower heat and add a splash of water if mushrooms begin to brown too quickly.

Add onion and another drizzle of oil; season with Italian Seasoning and a big pinch of salt. Cook, stirring, until veggies are browned and tender, 5-7 minutes. Taste and season with salt and pepper if desired. Turn off heat; transfer to a plate. Wipe out pan.

Meanwhile, in a small pot, heat cream sauce base over medium-high heat until gently bubbling, 1-2 minutes.

Reduce heat to medium; tear gouda into pieces and add to pot. Whisk until melted and combined.

Remove pot from heat; stir in half the mustard (all for 4 servings) until thoroughly combined.

Spread gouda sauce over flatbreads. Top with mushroom mixture and Parmesan. Return flatbreads to top rack until cheese melts and edges are golden brown, 6-10 minutes. (For 4 servings, return sheets to top and middle racks; swap positions halfway through baking.)

Cut flatbreads into pieces and divide between plates. Serve.