1. Brown Hamburger, drain, add taco seasoning mix and set aside.
2. Crush chips into the bottom of a 9x13 inch baking dish. Spread hamburger mixture over chips.
3. Layer onion, tomato and lettuce on top of hamburger.
4. In a large saucepan melt butter. Stir in flour and stir continuously until smooth. Stir in milk. Bring to a boil - stirring constantly and boil one minute. Add Velveeta Cheese and stir until melted. Pour sauce over casserole.
5. Bake at 400 degrees for 10-15 minutes or until bubbly.