Place four tablespoons butter, onion, garlic and mushrooms in a large skillet over medium-high heat; sauté until vegetables are tender, ~8 minutes, stirring often; transfer the mixture to a large bowl.
In same skillet, on medium-high heat, place remaining four tablespoons butter and flour, whisk ~2 minutes; stir in cream, milk, salt, and pepper; whisk ~5 minutes until mixture thickens; add sauce to the bowl; stir.
Preheat oven to 350°F. Bring a large pot of water to a boil; add green beans and boil 7 minutes; drain.
Add green beans and ¾ of the fried onions to the bowl and stir well; transfer all to a casserole or other baking dish; bake, uncovered, for 30 minutes, or until center is bubbly.
Remove from the oven and top with remaining fried onions; bake 5 minutes more.