Preheat the oven to 350 degrees F (175 degrees C). Spray a casserole dish with cooking spray.
Bring a large pot of lightly salted water to a boil. Add spaghetti and cook for 3 to 5 minutes or until al dente; drain.
Meanwhile, heat olive oil in a large skillet over medium-high heat until it begins to smoke. Add venison and cook until well browned, about 5 minutes. Stir in onion and green pepper, and continue cooking until softened, about 4 minutes. Add tomato paste and tomato sauce, season with garlic salt, dill, marjoram, and Italian seasoning. Bring to a boil, then reduce heat to medium and simmer for 5 minutes.
Place drained pasta into a prepared casserole dish and pour venison mixture overtop. Sprinkle with mozzarella and Parmesan cheese. Bake in the oven until cheese is bubbly and browned, about 25 minutes.