Heat olive oil in a large Dutch oven over medium heat. Add diced onion and minced garlic. Sauté until onions are softened and translucent (about 3-5 minutes). Be careful not to burn the garlic.

Add Italian sausage and ground beef, cooking until browned. Use a wooden spoon to break up the meat as it cooks.

Once the meat is browned, drain any excess grease from the pot.

Return the pot to the heat and add crushed tomatoes and tomato sauce. Add a little water to the jars, shake, and pour into the pot.

Stir in tomato paste, brown sugar, salt, pepper, dried basil, Italian seasoning, fennel seeds, and red pepper flakes (if using).

Reduce heat to low and simmer for 1.5 to 2 hours, stirring occasionally, skimming fat from the surface as needed. Taste and adjust seasoning as necessary.

Use this rich sauce for spaghetti, lasagna, or other pasta dishes.