Add the butter and olive oil to a large saucepan and melt over medium-high heat.
Place the chopped onions, celery, and carrots. Sauté for 2 minutes.
Add the minced garlic, ground beef, and Italian sausage to the vegetables and cook until the meat is no longer pink, about 5 minutes.
Add the salt, pepper, Italian seasoning, and red pepper flakes to the meat and stir to combine.
Then stir in the milk and allow the mixture to simmer for about 5 minutes.
Add the crushed tomatoes, tomato sauce, tomato paste, and beef broth. Bring the mixture to a boil, then lower the heat and simmer for 15 minutes.
While the Bolognese sauce is simmering, prepare the spaghetti noodles according to the package directions.
Serve! I like to mix some of the sauce with the spaghetti to keep the noodles from drying out, then serve more sauce on top.