Cook spaghetti noodles in a large pot of salted water, according to package instructions, until al dente. Drain and set aside.
Preheat oven to 350F.
Meanwhile, in a large skillet, combine Italian sausage, ground beef, and onion. Cook until meats are no longer pink and onion is tender; drain off any excess oil.
Add marinara sauce to the skillet and stir to combine; set aside.
In a medium bowl, beat together ricotta, cream cheese, and egg.
Spoon 1 cup of the meat sauce into the bottom of a 9x13 baking dish. Layer half of the spaghetti noodles, half of the meat mixture, half of the ricotta mixture, and half of the mozzarella and parmesan cheeses. Repeat layering for the remaining half of the ingredients.
Bake for 30 minutes until cheese is fully melted and the sauce is bubbling up around the sides of the dish. If desired, broil for 2-3 minutes to brown the top of the cheese.
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Notes
It's important to only cook the noodles al dente as they will continue to cook once added to the others ingredients and baked in the oven. You don't want soggy noodles.
For even faster assembly, stir together the cooked spaghetti and the meat sauce and pour mixture into the pan. Dollop the ricotta mixture throughout the spaghetti and then top everything with the shredded cheeses and bake.
Store leftover million dollar spaghetti, covered, in the refrigerator for 4-5 days or freeze. Thaw overnight and reheat in the oven or microwave.
Leave the casserole to cool for about 15 minutes if you would like to cut it into squares. If your family can’t wait, it scoops out just fine straight from the oven!