Gather all ingredients. Melt the butter in a small saucepan. Add the diced mushrooms and cook for 5-6 minutes, until liquid is mostly reduced. Add the flour and mix it together. Add the chicken broth and stir to combine.
01.
Mix in milk and then the seasonings. Whisk continuously and bring to a gentle boil. Stir continuously while it bubbles for about 4-6 minutes, it will thicken and reduce. Remove from heat.
02.
In a large bowl, mix together homemade cream of mushroom soup, green beans, cheese and milk, and half of the onions. Pour in a 9 x13 inch baking pan.
03.
Bake at 350 degrees for 25 minutes or until bubbly. Top with remaining french fried onions and bake for 5 more minutes.
04.