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 cut onion into 1/2-inch-thick wedges. Slice top off green pepper; core and deseed, then cut into 1/2-inch-thick rings.

Toss onion and green pepper on a lightly oiled baking sheet; season with salt and pepper. Add sausage to same sheet. Roast on top rack, flipping halfway through, until veggies are tender and sausage is cooked through, 15-17 minutes. (For 4 servings, you may need to divide between 2 sheets.)

Meanwhile, in a medium microwave-safe bowl, combine 1/4 cup water and 2 TBSP butter (1/3 cup water and 3 TBSP butter for 4 servings). Microwave until water is hot and butter has melted, 1 minute. Whisk in tomato paste until smooth. Season with salt and pepper.

Once veggies and sausage are roasted, remove from baking sheet. Place flatbreads on same sheet. Toast on top rack of oven until golden, 3-4 minutes. (For 4 servings, divide between 2 baking sheets; toast on top and middle racks.) Meanwhile, slice sausage on a diagonal into 1/2-inch-thick pieces.

Once flatbreads are toasted, remove sheet from oven and heat broiler to high. Evenly spread flatbreads with sauce, then top with mozzarella, Monterey Jack, sausage, and veggies. Season with salt and pepper. Broil until cheeses have melted and flatbreads are golden brown, 3-5 minutes. (For 4 servings, broil in batches.) TIP: Watch carefully to avoid burning.

Allow flatbreads to cool slightly, then slice into pieces and serve.