Heat the oil in a skillet over medium heat. Add the onion and sauté until starting to soften, 2-3 minutes. Stir in the garlic. Add the mushrooms and zucchini and sauté until softened, another 5-6 minutes. Season the mixture to taste with salt and pepper.
Stir in the Italian sausage and cook until browned, stirring often. Stir in the tomatoes, chicken broth, and marinara sauce and bring to a boil. Gently stir in the tortellini, nestling into the liquid. Season with oregano. Cover and cook for 7-10 minutes, stirring every few minutes, until tortellini is tender and cooked through. Remove the mixture from heat and stir in spinach until wilted.
Preheat the broiler. Sprinkle the skillet with cheese then place under the broiler for 5 minutes or until cheese is melted and starting to brown. Sprinkle with basil and serve immediately.
