Adjust rack to top position and preheat oven to 450 degrees. Wash and dry all produce. Halve, peel, and cut onion into 1/2-inch-thick wedges. Slice top off green pepper. Core, then cut into 1/2-inch-thick rounds.

Toss onion and green pepper on a lightly oiled baking sheet; season with salt and pepper. Add sausage to 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, add 1/4 cup water and 2 TBSP butter (1/3 cup water and 3 TBSP butter for 4 servings) to a medium, microwave-safe bowl. Microwave until water is hot and butter has melted, about 1 minute. Whisk in tomato paste until smooth. Season with salt and pepper.

Once roasted, remove veggies and sausage from baking sheet. Place flatbreads on same sheet. (For 4 servings, divide between both baking sheets as you did before.) Toast on top rack of oven until golden, 3-4 minutes. Meanwhile, slice sausage into 1/2-inchthick pieces on a diagonal.

Once flatbreads are toasted, remove from oven and heat broiler to high or oven to 500 degrees. Evenly spread flatbreads with sauce, then top with cheeses, sausage, and veggies. Season with salt and pepper. Broil or bake until cheeses have melted and flatbreads are golden brown, 3-5 minutes. (For 4 servings, broil or bake in batches.) TIP: Watch carefully for any burning.

Allow flatbreads to cool slightly, then slice into pieces. Serve.