For the fried shallots (onions):

1. Line a tray with 3 - 4 layers of paper towels.
2. Toss onions with flour and salt.
3. Heat about one inch of oil in a heavy skillet or large pan to 350° F. (See Note 2)
4. If you don’t have a thermometer, to test the oil put in 1 shallot ring. If it sizzles immediately, the oil is ready.
5. Fry the shallots in about 4-5 batches, don’t overcrowd the pan. Fry until light golden brown. (The time will depend on the heat of the oil, the type of pan, and the size of the shallots, about 2 minutes.)
6. Transfer to paper towel covered tray in a single layer, spread out. Let cool completely without covering or putting into a closed container.
7. The shallots can be made a few weeks in advance. Let them cool completely then store in an airtight container at room temperature. Do not refrigerate.
8. Preheat oven to 375 degrees.

To cook the green beans:

1. Bring a large pot of water to a boil. Cook green beans until just tender, about 5 - 6 minutes, or to your taste. Meanwhile fill a large bowl with ice water.
2. Drain green beans and put into the bowl of ice water. Once chilled, drain green beans well from ice water.

To make the creamy mushroom sauce:

1. In a large skillet melt butter over medium-high heat. Add mushrooms and sauté. Cook with a lid on for the first few minutes so they release their moisture, then remove the lid and cook until the liquid has evaporated and the mushrooms are golden brown, about 8 minutes. Add garlic and sauté 1 minute longer.
2. Sprinkle in flour and stir while cooking for about 30 seconds. Slowly whisk in the chicken broth and half and half.
3. Bring mixture to a simmer and let simmer a few minutes until thickened. If you run a finger along the back of the cooking spoon, it should leave a path.
4. Turn off the heat and slowly whisk in parmesan.
5. Salt and pepper to taste.

To finish the casserole:

1. Combine the mushroom sauce and the green beans and toss to evenly coat.
2. Optional: Crunch up a handful of the fried shallots and stir them into the beans.
3. Sprinkle with crispy fried shallots (or onions).
4. Cover with a sheet of foil and bake 20 minutes, uncover and bake until bubbly, about 10 minutes longer.
5. If the fried shallots begin to get too brown, tent a piece of foil on top of the casserole dish.
6. Serve warm.