Preheat oven to 400F.
Add water to a large pot over medium-high heat, once the water starts to boil, cook the tortellini by following the recommended time on the package, usually 5-7 minutes. Cook 1-2 minutes less the recommended time. Drain and return the tortellini to the pot. Add the pesto and stir gently until combined.
In the meantime, place a large skillet over medium heat, and add two tablespoons of olive oil.
Slice the sausages and add it to the hot skillet, cook for 2-3 minutes, stirring often, until the sausages start to brown. Remove skillet from heat and set aside.
Add the browned sausages, tomatoes, spinach and cheese to the pot with tortellini, stir well, but gently not to crush the tomatoes or the tortellini. Save a little cheese, about 1 cup to top the casserole.
Add a little olive oil to a 9 inch casserole dish, about 1 tablespoon, and add the tortellini mixture to the casserole, top with the remaining cheese and bake for 20-25 minutes, until the top layer starts to look golden - light brown.