Heat the oil in a large skillet or Dutch oven over medium heat. Add the beef and onions, spreading them evenly over the skillet, and cook undisturbed for several minutes. Stir, breaking up the beef, and continue to cook until beef is browned and onion is softened, about 8 minutes. Add the garlic and stir until fragrant. Season to taste with salt and pepper. Drain any excess fat from the pan.

In a slow cooker, combine the crushed tomatoes, paste, basil, parsley, 2 Tablespoons sugar, oregano, red pepper flakes, and salt. Stir in the ground beef mixture and taste. Add salt, pepper, and additional sugar, to taste.

Cover and cook on low for 8 hours or on high for 4.