Heat the olive oil in a large skillet over medium-high heat.
Add the ground beef, onion, and garlic. Cook, crumbling the meat until no longer pink.
Drain the grease from the meat and add to the slow cooker.
Salt & pepper the meat to taste. Add in the Italian seasoning, sugar, and red pepper flakes.
Pour the cans of tomato sauce, and the can of crushed tomatoes over the meat and stir.
Place the lid on the slow cooker and cook on low for 6-8 hours or high for 3-4 hours.
Serve immediately with your favorite pasta.
Store leftovers in the refrigerator for up to 4 days.
NOTE ON FREEZING: Once the sauce is cooled, you can store it in an airtight zip-top bag. Lay it flat in the freezer and store for up to 3 months. Allow the sauce to thaw for 8-10 hours in the refrigerator before reheating. When heating it back up, you can simmer on low on the stove for around 30 minutes, or just throw it in the slow cooker again on low for a few hours until it's warmed throughNOTE: A double batch of this sauce will fit in a 6 quart or larger slow cooker.