Position oven rack in center of oven and preheat to 350 degrees.
Crumble sausage meat into a large saucepan and brown over medium heat, stirring often, about 4 minutes.
Drain off any fat, then add the onion and bell pepper. Cook, stirring often, until softened, about 3 minutes.
Stir in tomatoes, peas, tomato paste, oregano, basil, thyme, fennel seeds, salt and pepper. Bring to a simmer, then reduce the heat and cook uncovered 5 minutes, stirring often.
Stir in cooked pasta and half the cheese. Spread evenly into a 9x13-inch baking pan. Top evenly with remaining cheese.
Bake until cheese has melted and casserole is bubbling, about 20 minutes. Let stand 10 minutes at room temperature before slicing into 8 pieces.