In a medium pot, heat 4 to 6 quarts of water to a rolling boil, add 1-teaspoon salt. Add uncooked pasta and stir. Return to a boil and cook for about 12 minutes, uncovered, stirring occasionally. Remove from heat and drain well. Immediately rinse pasta with cold water until cool to the touch. Drain well, cover and set aside.
Heat olive oil in a Dutch oven or large pot over medium heat. Add Italian sausage and brown for about 5 minutes. Make sure to crumble the sausage as you stir to breakdown any large chunks. Add onions and sauté until translucent, about 5 minutes. Add garlic, oregano, and red pepper flakes. Cook for about a minute, or until garlic is fragrant, but not browned. Stir in the tomato paste and continue cooking and stirring until the tomato paste has darkened in color to a brownish red.
Stir in diced tomatoes, chicken stock and oregano. Stir all the way to the bottom making sure to incorporate the tomato paste. Bring tomato mixture to a boil and then reduce heat and simmer for 30 to 40 minutes, stirring occasionally. Adjust seasoning with salt and pepper to taste.
In a small bowl, stir together ricotta cheese, Parmesan, 1-cup mozzarella cheese, salt, and pepper.
Right before serving, stir in desired amount of pasta into the heated soup and stir to combine.
Place a scoop of the cheese mixture in each soup bowl and ladle soup on top. Sprinkle with more mozzarella and fresh sliced basil, if desired.