In a large skillet over medium-high heat, cook the chorizo, breaking it up with a spoon, until browned and fully cooked, about 6-8 minutes. Add the onion and garlic, cooking for another 3 minutes until softened. Drain excess grease and transfer to your slow cooker.

Add the diced green chiles, Monterey Jack cheese, Oaxaca cheese, cream cheese cubes, heavy cream, and cumin to the slow cooker. Stir to combine.

Cover and cook on low for 1½ to 2 hours, stirring every 30 minutes, until all the cheese is melted and the dip is smooth and creamy.

Stir in the chopped cilantro and switch the slow cooker to the "warm" setting.

Top with diced tomatoes and extra cilantro if desired. Serve directly from the slow cooker with plenty of tortilla chips for dipping.