Roasting the peppers before use brings out their natural sweetness and removes their thin waxy skin. Martha made this recipe on Martha Bakes episode 608.
Heat oven to 500 degrees, with a pizza stone placed on the lowest rack, for at least 30 minutes. Place parchment paper on a pizza peel.
Mix tomatoes, garlic, and 2 teaspoons olive oil in a medium bowl until well combined; season with salt and black pepper, and red pepper flakes.
Cook peppers over an open flame (on the stove or a grill) until skin is blistered and charred, 8 to 10 minutes. Transfer to a large bowl and cover tightly with plastic wrap; let steam 10 minutes. Once peppers are cool enough to touch, rub skin off using a paper towel. Remove stems and seeds; dice peppers.
Heat 1 tablespoon oil in a large skillet over medium-high. Add sausage and cook, stirring occasionally, until browned, about 8 minutes. Transfer to a plate.
Add remaining tablespoon oil and onion to pan. Cook, stirring up brown bits, until golden, about 3 minutes. Return sausage to pan along with roasted peppers and tomato sauce. Bring to a simmer and cook until slightly thickened, about 5 minutes. Season with salt and more crushed red pepper flakes, if desired.
On a lightly floured work surface, roll each piece of dough into a 1/8-inch-thick circle, about 12 inches in diameter, with the edges of the dough slightly thinner than the center.
Place one-quarter of the filling on half of each round. Top with one-quarter of the mozzarella and season with salt and pepper. Fold dough in half over filling and crimp as desired to seal. Place 2 calzones on prepared pizza peel and transfer to pizza stone along with parchment paper. Bake until dough is puffed and golden, and filling is hot, 10 to 15 minutes. Repeat with remaining 2 calzones. Serve immediately.