Cook noodles according to package instructions.

Drain the cooked noodles.

Chop the onion.

Melt margarine in a pan.

Saute the chopped onion in melted margarine until softened.

Stir in flour to create a roux.

Add tomato juice, mushrooms (with their juice), sugar, salt, and pepper to the pan.

Simmer the sauce for a few minutes, stirring occasionally.

Grease a 9x13 inch pan or a 2-quart casserole dish.

Place the cooked noodles into the greased pan.

Add cubed Velveeta cheese over the noodles.

Pour the tomato juice mixture over the noodles and cheese.

Bake in a preheated oven at 350°F (175°C) for 45 minutes.

Bake until the top is bubbly and lightly browned.