Wash and clean all the mushroom, remove stems (save for other dishes maybe), then slice or cut into pieces.
In a sauce pan, melt 1 Tbsp butter, then add crushed garlic. Stir quickly to let it brown and start giving off aroma.
Now add the mushroom pieces, lightly season with some salt and freshly ground pepper. Stir until mushroom soften up a bit.
In a separate bowl, combine the flour with chicken stock and dry sherry until fully mix (no lump).
Pour this mixture onto mushroom, stir for a minute to fully mixed and reduce heat to simmer.
Add the heavy cream and keep simmering til thickening (about 10 minutes).
Taste test and adjust salt accordingly.
Stir in the drained cook green bean, make sure they're all evenly cover with sauce.
Remove from heat. From here on you can chill this until ready to serve or bake immediately.