In a skillet over medium-high heat, brown meat and onions, breaking meat up as it cooks. Cook until meat is cooked through and onions are translucent. Add garlic and continue to cook, stirring, for 1 minute. Add tomato paste and cook, stirring, until incorporated. 

Add meat mixture to slow cooker along with crushed tomatoes, tomato sauce, diced tomatoes, sugar, oregano, thyme, red pepper flakes, and bay leaf.

Cover slow cooker and cook on high for 3-4 hours or on low for 6-8 hours. Immediately before serving, stir in basil, parsley, and butter. Stir until butter melts. Taste, and add salt if desired.

Serve over cooked pasta of your choice.