Preheat the oven to 350 degrees F.

In a large cast iron skillet, heat the olive oil over medium heat and then sauté onions until translucent, about 5 to 7 minutes. Add the garlic and continue to sauté for about a minute longer.

Add the beef / venison and Italian sausage and break it up with a wooden spoon. Sauté until cooked through, about 5 to 7 minutes. Add oregano and red pepper flakes to the skillet and cook for one more minute.

Stir tomatoes, spaghetti pieces, water, and salt into the beef mixture. Cover the skillet and cook, stirring often for about 7 minutes, or until the spaghetti has soaked up a good bit of the water. Remove the cover and allow to continue cooking on low heat for about 7 minutes longer, or until the spaghetti is fully cooked.

Add ¼ cup of the sharp cheddar cheese, ¼ cup of the mozzarella cheese, and ¼ cup of the parmesan cheese to the skillet and gently stir the cheese into the spaghetti until incorporated. Add the remainder of cheese over the top of the skillet and place in the oven for about 15 to 20 minutes or until bubbly. Remove spaghetti from the oven and allow it to rest for about 5 minutes. Sprinkle onions over the top and serve.