Microwave spinach in a large glass bowl at HIGH 2 minutes or until spinach wilts. Drain and pat dry with paper towels; coarsely chop.
Melt butter in a large saucepan over medium heat. Add onions and garlic; sauté 2 minutes. Gradually add milk and cheese sauce mix, stirring well with a whisk. Bring to a boil; reduce heat, and simmer 3 minutes, stirring constantly. Add cubed cheese, tomatoes, and ground red pepper; cook 3 minutes or until cheese melts, stirring constantly. Remove from heat. Stir in spinach. Serve warm.