Béchamel sauce (known as one of the "mother sauces"). If can learn to make good Béchamel--and just about anyone can--you can add a lot of variety to your everyday cooking with very little extra effort and time. Here's a basic recipe:
2 Tbs. butter
2 Tbs. all-purpose flour
1 cup milk
salt and pepper to taste
In a small sauce pan, melt butter over low heat. Add flour and whisk until smooth, being sure to get all the flour out of the corners of the pan. Cook until a light tan color, whisking constantly—about 3 to 5 minutes. Gradually add the milk and continue to whisk over medium high heat until mixture is thickened and coats the back of a spoon, about 4 minutes. Add salt and pepper to taste.
So what can you do with this simple white sauce? By itself it's the traditional sauce for the layers in lasagna, and I also use it as the sauce on a breakfast pizza (more on that in another post, I promise!). Add Gruyère or parmesan and you've got Mornay sauce for an upscale mac and cheese. Stir in prepared mustard, cheddar and stout and you can serve Welsh rarebit. A Béchamel with minced onions, white wine, sage and rosemary can accompany pork chops. Or you can make an awesome crab meat pizza.
Regular onions would have worked, too, or shallots. The celery pairs perfectly with the sauce, so I wouldn't swap that out, but I suspect artichoke wouldn't be out of place here, nor crumbled bacon. If you're not sure how a particular topping will pair with this dish, dip it in the sauce and have a taste.
Give Béchamel sauce a try for a seafood pizza, and then start experiementing on variations with other dishes. You'll be opening up a whole world of flavors, and when you realize how simple and economical this versatile sauce is, you'll never buy a packaged sauce again.