Cook the pasta according to package instructions. Drain and transfer to a large bowl.
Preheat the oven to 350°F (175°C).
In a large skillet over medium heat, crumble the Italian sausage and brown it for about 5 minutes until no longer pink. Use a wooden spoon to break up the sausage for even cooking.
Add the chopped onion and green bell pepper to the skillet and cook for another 3 minutes until softened.
Stir in the diced tomatoes, tomato paste, oregano, basil, thyme, fennel seed, salt, and pepper. Add the chicken broth and bring to a boil. Reduce heat and simmer for 20 minutes, stirring occasionally.
Stir in the frozen green peas and remove the sauce from heat.
Pour the sauce over the cooked pasta in the large bowl and stir to combine. Mix in 1 cup of shredded mozzarella cheese.
Transfer the pasta mixture to a large casserole dish and top with the remaining mozzarella cheese.
Bake in the preheated oven for about 20 minutes, or until the cheese is melted and the casserole is bubbling.
Garnish with parsley or basil if desired, and serve warm.
Notes
For a spicier kick, use hot Italian sausage instead of mild.