1. Preheat oven to 425 degrees. Melt butter in a large saucepan over medium heat. Add onion and garlic. Cook and stir until onion is crisp-tender and garlic is fragrant, 2-3 minutes.
2. Stir in flour; cook and stir an additional minute. Add half-and-half and cook, stirring constantly, until mixture boils and thickens, about 2 minutes.
3. Remove saucepan from heat. Stir in chopped spinach, water chestnuts, Cheddar and vegetable soup mix. Spoon dip into a 1 quart casserole or gratin dish. Sprinkle with parmesan.
4. Bake dip until it is bubbly and cheese is melted, 10-15 minutes.
