Bring the water to boil in a large pot or dutch oven. While it comes to a boil, prep the beans. Fill a large bowl with ice water. Add the salt and beans to the boiling water. Cover and cook 6 minutes or until crisp-tender and still bright green. (Be sure to cook the beans to your desired level of doneness; from here on they will re-heat but won’t cook further). Drain beans in a colander, then plunge into ice water to stop the cooking. Drain a bit in the colander again. Place a double layer of paper towels on a baking sheet (or use clean kitchen towel), arrange beans in a single layer to dry, top with a double layer of towels and pat dry. Let continue to dry while finishing.

Clean the mushrooms; break off and discard stems. Break the mushroom tops into pieces or roughly chop. Melt the butter in a skillet over medium heat. Add the mushrooms, garlic, salt, and pepper. Stirring often, cook until mushrooms begin to soften and exude their liquid, about 6 minutes.

Stir in flour and cook 1 minute. A tablespoon at a time at first, add the chicken stock and sherry; bring to a simmer. Add the half & half, simmer until sauce thickens, about 10 to 15 minutes. Taste and adjust seasonings.

Stir in cooked beans and half the can of fried onions.

To bake immediately- Preheat oven to 425°F. Transfer hot bean mixture to a greased baking dish (about the size of a 8″ or 9″ square pan- double recipe to fill a 9″x13″ pan). Bake uncovered for 10 minutes, remove from oven and add the remaining fried onions, return to oven and cook an additional 5 minutes or until fried onions are golden brown.

To bake later- Transfer hot bean mixture to a greased baking dish (about the size of a 8″ or 9″ square pan- double recipe to fill a 9″x13″ pan). Do NOT top with fried onions yet. Let the dish cool to room temperature, cover with plastic wrap and refrigerate. Return to room temperature before baking (leave on counter about 2 hours). Remove plastic wrap. Heat in 425°F oven until hot and bubbly, about 20-30 minutes. Add topping and bake for another 5 minutes or until fried onions are golden brown.