Preheat the oven to 350°F (175°C).
Heat a large skillet over medium heat. Add the chorizo and cook, breaking it up with a spoon, until no longer pink, about 5–7 minutes; drain any excess fat.
Stir the marinara and diced tomatoes into the cooked chorizo and bring to a simmer. Cook 3 minutes to reduce excess liquid.
Cook the tortellini according to package directions until al dente, then drain.
Toss the drained tortellini with the sauce in the skillet until evenly coated.
Spoon a thin layer of sauce across the bottom of a casserole dish. Add half the sauce-coated tortellini, then top with half the pepperoni, olives, green pepper slices, red onion, and half the mozzarella.
Repeat with the remaining tortellini, pepperoni, olives, peppers, onion, and cheese to form a second layer.
Bake in the preheated oven for 25 minutes until heated through and cheese is melted. If you want browned cheese on top, broil for up to 5 minutes watching carefully.
Remove from the oven and let sit a few minutes before serving.