Preheat oven to 400°F. In a large nonstick, ovenproof skillet, warm olive oil over medium-high heat. (If skillet isn't ovenproof, wrap handles in a double layer of foil.) Remove sausages from casings and crumble meat into skillet. Cook, stirring and breaking up large pieces, until there are no traces of pink, about 10 minutes.
Squeeze as much liquid as possible from spinach. Pour excess fat from skillet; add spinach and red peppers. Cook, stirring well and scraping up any cooked bits on bottom of skillet.
Pour eggs into skillet and stir to mix with spinachsausage mixture. Stop stirring and cook over medium heat, lifting edges of frittata with a spatula to let uncooked eggs flow underneath. Cook until almost set, about 3 minutes.
Sprinkle Parmesan on top and transfer skillet to oven. Bake, uncovered, until frittata is puffed and lightly browned on top, 8 to 10 minutes. Lift frittata around edges of skillet and gently shake pan to loosen. Slide out of pan and onto a cutting board and cut into wedges. Serve hot or at room temperature.