Heat olive oil over medium high heat in a skillet or stock pot. Add Italian sausage and brown on all sides.
Add water and salt to a large stock pot and boil on medium high. Once water reaches a rolling boil, add tortellini. Cook according to package instructions, usually 8-10 minutes, or until pasta is al-dente. Once they are done, drain and set aside.
When Italian sausage is browned, remove from pan and set aside to cool on a cutting board.
Add the onion trio to the same pan, sauté for 5 minutes. Add garlic and sauté for 1 minute. Add homemade marinara sauce and turn heat down to medium low to allow sauce to simmer.
Once Italian sausage has cooled, slice it to 1/2 inch thick slices. Add it back to the pan with the sauce. Simmer for 10 minutes. Add tortellini pasta to the pan and combine.
Turn oven on broil. Transfer pasta to an oven safe dish (if not already using one). Sprinkle cheese over the top and broil until top cheese is bubbly and browned. BE SURE TO WATCH IT BECAUSE THE BROILER WILL BURN THE DISH VERY QUICKLY.