Russian Cabbage Rolls with Gravy

Name: Russian Cabbage Rolls with Gravy
Ingredients: 1 medium head cabbage; 2.5 cups water, divided; 0.75 cup uncooked white rice; 3 tablespoons vegetable oil, divided; 2 onions, chopped, divided; 1 pound ground beef; 0.75 pound ground pork; salt and ground black pepper to taste; 3 carrots, grated; 3 tablespoons tomato paste; 2 tablespoons sour cream; 1 teaspoon dried mixed herbs, or as needed