Preheat oven to 450 degrees. Halve, peel, and cut onion into 1/2 inch wedges. Core and slice the pepper into 1/4 rings. Cut the mushrooms into 1/4 slices.
Toss veggies on a lightly oiled baking sheet and season with salt and pepper. Add the sausage to the same sheet (whole). Roast for 15-20 minutes, flipping halfway through, until the veggies are soft and sausage is fully cooked.
In the meantime, put 2 tbsp butter in a small microwave safe bowl. Microwave for 30 seconds, until the butter melts, then mix in 1/4 cup water and tomato paste until smooth. Season with salt and pepper.
Once the veggies and sausage are roasted, remove them from baking sheet and put your flatbreads on the same sheet. Toast the flatbreads on the top rack for 3-4 minutes until slightly browned.
Remove the flatbreads from the oven and slice your sausage into 1/2 inch pieces. Spread the flatbreads with the tomato sauce mixture, then top with mozzarella, Monterey Jack, veggies, and sausage.
Broil on high for 3-5 minutes until golden brown. Watch carefully to avoid burning! Speak from many, many experiences :)
Allow the flatbreads to cool - slice and enjoy!