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 ½-inch-thick wedges. Slice top off green pepper; core and deseed, then cut crosswise into ½-inch-thick rings.

Place flatbreads on a lightly oiled baking sheet; toast on top rack until golden, 5-7 minutes. (For 4 servings, divide between 2 sheets; toast on top and middle racks.)

While flatbreads toast, heat a drizzle of oil in a large pan over medium-high heat. Add onion, green pepper, salt, and pepper. Cook, stirring occasionally, until browned and softened, 6-8 minutes. Turn off heat; transfer veggies to a plate. Wipe out pan.

Remove sausage* from casing if necessary; discard casing. Heat a drizzle of oil in same pan over medium-high heat. Add sausage to pan and cook, breaking meat up into pieces, until browned and cooked through, 4-6 minutes.

Meanwhile, in a medium microwave-safe bowl, combine 2 TBSP water and 2 TBSP butter (4 TBSP 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 flatbreads are toasted, remove sheet from oven. Evenly spread flatbreads with sauce, then top with mozzarella, Monterey Jack, sausage, and veggies. Season with salt and pepper.

Bake until cheese melts and flatbreads are golden brown, 3-5 minutes.

Allow flatbreads to cool slightly, then slice into pieces.

Divide between plates and serve.