I don't recommend to skip cream of mushroom soup because it adds flavor. To replace it, sprinkle 2 tablespoons of cornstarch or 4 tablespoons of flour over sautéed veggies, and stir. Then add 1/3 cup heavy cream or 1 cup any milk, stir to incorporate, bring to a boil and simmer until roux has thickened. Proceed with the recipe.