Prepare the sauce by combining crushed tomatoes, tomato sauce, salt, red chili flakes, brown sugar, red wine vinegar, and grated garlic in a large bowl.
Mix well and set aside.
Soften the cabbage leaves by boiling in a large pot of salted water for 2-4 minutes until flexible.
Remove and drain them on a towel.
In a mixing bowl, combine ground beef, chopped onion, cooked rice, parsley, egg, and black pepper.
Mix this filling until everything is blended together.
Take a cabbage leaf, place about 1/2 cup of filling at the leaf's center, and fold in the sides.
Roll up the leaf and secure it seam-side down.
Layer the cabbage rolls in a slow cooker and pour the sauce over them.
Cook on low heat for 6-8 hours.