Bring the water to boil in a large pot or Dutch oven. Add the salt and beans to the boiling water. Cover and cook for 6 minutes or until the beans are crisp-tender and still bright green. Drain the beans in a colander, then plunge into ice water to stop the cooking. Drain again and dry the beans.
Melt the butter in a large skillet on medium heat. Add the mushrooms, garlic, salt, and pepper. Cook until the mushrooms begin to soften and exude their liquid, about 6 minutes. Stir in the flour and cook for a minute. Gradually add the chicken stock and sherry; bring to a simmer. Add the half & half and simmer until the sauce thickens, about 10 - 15 minutes. Stir in the cooked and dry beans.
In a food processor, process the bread, butter, and seasonings in about 10 quick pulses. Stir in the onions.
Preheat oven to 425F/220C. Transfer the Green Bean-Mushroom Sauce mixture to a greased baking dish. Top with the Topping mixture and bake uncovered for 15 minutes.
