Preheat oven to 350 degrees F (175 degrees C).

In a large skillet over medium-high heat, brown the ground beef with the onions. Drain excess fat, and season with salt and pepper to taste.

Spread the beef mixture evenly over the bottom of a 2 quart casserole dish. Arrange tater tots evenly over beef layer. In a small bowl, stir the soup into the milk until smooth; pour over tater tot and beef layers. Sprinkle Cheddar cheese evenly over the top.

Bake in preheated oven for 30 to 40 minutes, until cheese is bubbly and slightly brown.