If you don't want to use a canned soup, simply place 1/2 cup onion, 1 garlic clove,  and 8 oz mushrooms into a food processor and pulse a couple of times until small chunks. Melt the 4 tablespoons of butter in a saucepan. Add in the vegetable mixture, season with salt and pepper to taste; cook for 5 minutes. Add in the 4 tablespoons of all-purpose flour and stir the ingredients together well for about a minute to let the flour cook a bit. Add in the 1/2 cup evaporated milk and 1/2 cup chicken broth; cook until thickened.