Preheat oven to 375 degrees F.
Melt butter in a large pan over medium-high heat.
Add in mushrooms, onions, and a pinch of salt. Cook until onions soften, about 6-8 minutes.
Once onions have softened, add in thyme, poultry seasoning, rubbed sage, and flour. Cook flour, stirring often about 2 minutes.
Slowly start pouring in heavy cream and whisk it in trying to remove any lumps. Continue whisking and begin pouring the beef broth.
Continue whisking and reduce the heat to medium-low. Allow the sauce to continue cooking and thickening for about 6 minutes or until sauce thickens.
Remove pan from the heat and fold in the green beans.
Pour green bean mixture into a well-greased 10x12 baking dish.
Sprinkle chopped pecans over the casserole and layer french fried onions over the casserole.
Bake for 25 minutes.
Allow casserole to cool for 5-10 minutes before serving.