Preheat oven to 375 F
Butter a 9 x 13" baking dish
In a large skillet, melt 3 tbls butter over medium heat
Stir in mushrooms and garlic, and saute for about 5 minutes
Add remaining 2 tbls butter and melt
Stir in flour until absorbed
Stir in milk, stirring constantly
Add salt, cayenne and pepper to taste
Bring to a simmer and stir until thickened
Stir in cheese until melted
Remove from heat and check for seasoning
Add frozen green beans and scallions and stir
Pour into baking dish
Arrange onion rings over the top
Bake in preheated oven for 30-40 minutes until golden and bubbly
