1In 2-quart microwavable casserole, mix milk and cheeses. Microwave uncovered on High 5 minutes, stirring every minute, until creamy.
2Meanwhile, in 10-inch skillet, melt butter over medium-high heat. Cook onion and garlic in butter 3 minutes or until tender. Add tomato; cook 2 minutes longer. Stir in chiles.
3Stir vegetable mixture into cheese mixture until blended. Spoon dip into 2-quart slow cooker. Keep warm on Low heat setting up to 2 hours, stirring occasionally. Serve with tortilla chips.