Hi Craig - this is an all purpose cheese sauce that you can use for your onions, or broccoli, cauliflower, etc. The thing to note is that the quality of the sauce will only be as good as the quality of the cheese you use.

Cook your vegetable separately (according to package directions), then top with the cheese sauce. If you want to do it ahead, it will work ok: put the veggies in a casserole dish, top with the cheese sauce and set aside (or covered in the fridge, but bring to room temp before continuing) until you want to serve. Then place it in a high oven or under the broiler until it's hot and bubbly.

Cheese Sauce

2 tbsp butter
2 tbsp flour
2 cups milk (whole milk makes a richer sauce, but 2% is ok)
4 oz. grated cheese (cheddar OR Gruyere or other good swiss cheese, depending on taste)
salt and pepper to taste

In a heavy saucepan, melt butter until bubbling and then add flour. Stir for 1 minute to allow flour to cook slightly (don't allow to brown). SLOWLY add the milk, a little at a time, stirring until completely incorporated (this is important - adding too much milk at once will cause lumps). Once the milk is added, bring to a simmer and continue to cook, stirring, until the sauce thickens. Add the cheese and stir until it is melted and thoroughly incorporated. Season with salt and pepper.

Hope this is what you are looking for. Happy Thanksgiving!