Brown beef with onion in skillet. Add garlic, taco seasoning, and tomato sauce. Simmer 5 minutes.
Melt butter, whisk in flour, cook 1 minute. Slowly add warm milk, stir until smooth. Melt in cheese. Season with salt & pepper.
Warm taco shells, fill with beef, lettuce, tomatoes. Drizzle with cheese sauce. Add sour cream and cilantro.
Plate immediately while hot and gooey.
