Dice the zucchini, bell pepper, and mushrooms. Grate the cheese. Stir together mozzarella and Swiss cheese.
Preheat the oven to 375°F (190°C).
Heat the oil in a large pot over high heat. Add ground beef in an even layer. Don´t stir for the first 1-2 minutes letting the beef brown, then start stirring and cook it for a couple of minutes or until no longer pink. Add Italian seasoning, garlic and onion powder. Cook for 30 seconds. Season with salt and pepper. Transfer to a plate.
Add the zucchini, bell pepper, and mushrooms to the pot. Cook for about 5 minutes over medium-high heat or until soft. Season with salt and pepper.
Add beef back to the pot. Add spaghetti sauce and cook over low heat for about 10 minutes. My sauce was quick thick. Add 1 1/4 cups of water and bring to a boil. Season with salt and pepper to taste. Add tortellini and bring it all once again to a boil.
Note: the extra water in the sauce is needed to cook the tortellini while the casserole is baking and for the casserole to not end up dry. If your sauce is more runny you may need to add less water - you can add it gradually and assess if you need more.
Add half of the tortellini with sauce to a 9x13 (23x33cm) casserole dish (the sauce shouldn´t be very soupy but it can´t be as thick as regular spaghetti sauce - you can check the photos in the body of the post for a reference).
Sprinkle 1/2 of the Swiss and mozzarella cheese on top.
Add the remaining tortellini and sauce.
Sprinkle with the remaining Swiss and mozzarella and then with the whole grated Parmesan.
Cover the dish tightly with a piece of aluminum foil and bake for 40 minutes.
Uncover and bake for a further 5-10 minutes or until the cheese is browned and the edges of the casserole bubbling and crispy. I´m using a fan setting for this step but you could also move the casserole dish under the burner (grill) and bake for 1-2 minutes or until the cheese is browned (make sure to not burn it).
Dig in and enjoy!