Ingredients:

some oil (~1/4 cup)
1 medium/large yellow/white onion
1/4 cup butter
1 tablespoon flour
some salt (~1/2-1 teaspoon depending on preference)
pinch of black pepper
1 teaspoon mustard
some milk (~1/2 cup)
approx. 1 cup shredded cheese (based on preference, I use cheddar)
some bread crumbs (~1/2 cup)

How to prepare:

Preheat oven to 350 degrees F.
Put oil in small pan and add onions. If preparing dip then grate the onions in a food processor. If preparing a side dish then simply slice onion into small pieces. Cook until soft and clear, but not brown.
While onion is cooking, melt the butter in a small saucepan over medium heat. Mix in flour, salt, pepper and mustard. Stir constantly. Gradually add in milk and continue stirring until thick. Add cheese and continue stirring until melted. (In the meantime watch for the onion and set aside once ready.)
Take a pie shaped pan and spread the onions. Spread the sauce on top. (Try to spread it as you pour it onto the onions as it will be too sticky to spread later.) Top with bread crumbs.
Bake for 20-25 minutes.

Makes a sidedish for 2-3 people or a good amount of dip that can be served with bread or crackers.