In a large skillet over medium-high heat, brown the Italian sausage with the diced onion, breaking it up into small crumbles as it cooks, about 8-10 minutes. Add garlic during the last minute. Drain excess grease.
Transfer the cooked sausage mixture to your slow cooker.
Add the cream cheese cubes, drained tomatoes, Italian cheese blend, mayonnaise, sour cream, oregano, and red pepper flakes (if using). Stir well to combine.
Cover and cook on LOW for 2-3 hours, stirring occasionally, until the cheese is completely melted and the dip is hot and bubbly.
Give it a good stir before serving and garnish with fresh basil or parsley.
Keep on WARM setting and serve with toasted baguette slices, crackers, or tortilla chips.