In small dish, pour boiling water over dried mushrooms and allow to sit at least 15 minutes to rehydrate dried mushrooms.
To blanch green beans, add kosher salt to boiling water, cook green beans in boiling salted water for about a minute, then immediately transfer to an ice bath. Once cooled, drain green beans and set aside.
In heavy bottom stockpot, melt butter over medium high heat and add mushrooms. Allow to brown, stirring only to prevent burning, for about 5-8 minutes. Sprinkle flour over mushrooms, stir, and allow to cook for another couple of minutes.
Add chicken broth to mushroom roux, stir well and scrape bottom to remove any bits that may have stuck to pan. Add rehydrated mushrooms along with any liquid. Reduce heat to medium low, add milk, stir, and continue to cook until gravy thickens.
Add green beans and 1/2 cup of the onion pieces. Stir well to combine. Top with remaining fried onion pieces.
Either refrigerate overnight to finish cooking the next day, or cook in 350 degree oven until heated through and onion pieces start to brown, 20-30 minutes.