In 2 quart shallow baking dish, arrange 18 Ritz crackers in 3 long rows. Combine spinach and 2 cups grated cheese; sprinkle half of mixture over crackers; repeat layers, top with remaining crackers; sprinkle with remaining cheese.
In medium bowl, beat together milk, eggs, mustard, liquid hot pepper seasoning and garlic. Pour evenly over mixture in baking dish. Refrigerate for 1 hour. Bake at 350 degrees for 1 hour or until puffed and golden. Cut into squares to serve.