Mushroom pesto sauce for pasta

We had this with Portobello mushroom ravioli that I bought at Costco; they are very flavorful, and next time I'll add some cheese ravioli to the mix as well.

In large skillet, sauté one large chopped onion until soft. Add one pound chopped mushrooms, sprinkle with salt, and continue to cook on medium-low heat until most of the liquid is evaporated.

Sprinkle the onions and mushrooms with one tablespoon flour, and stir, cooking, for about a minute. Slowly add 1-2 cups of skim milk, allowing it to thicken as you add it.

Stir in 2-3 generous tablespoons basil pesto, and serve over the pasta.