Bring the water to a boil. Cook tortellini according to package directions. Drain.
While you are waiting for your water to boil and your pasta to cook, cook sausage until cooked through, about 10 minutes. Transfer to a cutting surface, slice and set aside.
In a large skillet, heat olive oil over medium heat. Add onions and garlic. Cook just until softened, about 3 minutes. Stir in sliced sausage, cooked pasta, cherry tomatoes, spinach leaves, kosher salt and black pepper. Cook just until spinach wilts, about 2 minutes.
Transfer to a serving bowl, sprinkle with chopped basil and serve.