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

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

Heat a large drizzle of oil in a large, preferably nonstick, pan over medium-high heat. Add onion; cook, stirring occasionally, until lightly browned, 3-4 minutes.

Add mushrooms; cook, stirring occasionally, until veggies are softened, 5 minutes.

Add 1 tsp sugar (2 tsp for 4 servings) and a splash of water; cook until onions are jammy and caramelized and mushrooms are browned and tender, 2-3 minutes more. Season with salt and pepper. (For 4, you may need to work in batches.) 

Meanwhile, place flatbreads (dimpled sides down) on a baking sheet. Brush or rub each flatbread with a drizzle of olive oil; lightly season with salt and pepper. (For 4 servings, divide flatbreads between two sheets.)

Toast on top rack until golden brown, 3-5 minutes. (For 4, toast on top and middle racks, swapping rack positions halfway through.)

Once flatbreads are toasted, remove sheet from oven. Carefully flip (dimpled sides up) and sprinkle with mozzarella. Top with caramelized onion and mushrooms. 

Return to top rack and bake until cheese melts, 4-6 minutes. (For 4 servings, bake on top and middle racks, swapping rack positions halfway through.)

Meanwhile, in a large bowl, toss arugula with as much creamy balsamic dressing as you like. 

Taste and season with salt if desired.

Cut flatbreads into pieces.

Divide flatbreads between plates and top with as much salad as you like. Serve with any remaining salad on the side.