Slow-Cooker Stuffed Cabbage Rolls

Bring a large pot of water to a boil. Add cabbage and boil until the leaves have softened a bit, 2 to 3 minutes. Transfer the cabbage to a plate or shallow bowl. Let cool slightly. When cool enough to handle, gently pull 12 leaves from the cabbage head. Chop the remaining cabbage (you should have about 1 cup). 

Transfer the chopped cabbage to a large bowl. Add beef, onion, rice, egg, 3 cloves minced garlic, 1 1/4 teaspoons Italian seasoning, 1/2 teaspoon salt and pepper. With clean hands, mix until just combined. Arrange 1 cabbage leaf on a work surface. Cut out the thickest part of the stem in the center, keeping the leaf intact. Place a generous 1/3 cup beef mixture in the center. Fold both sides over the filling and roll up. Repeat with the remaining leaves and filling. 

Stir crushed tomatoes, diced tomatoes, the remaining 3 cloves minced garlic, 1 1/2 teaspoons Italian seasoning, 1/2 teaspoon salt, crushed red pepper and sugar (if using) together in a medium bowl. 

Spread 1 cup sauce over the bottom of a 6- to 8-quart slow cooker. Line the bottom with cabbage rolls, seam-side down. Pour half of the remaining sauce over the rolls. Place the remaining rolls on top and pour the rest of the sauce over them. Cover and cook on High for 4 hours or Low for 8 hours.