• To blanch fresh beans: In a large saucepan, bring a gallon of water and 1 tablespoon of salt to a boil.
• Add the beans and blanch for 5 minutes. Drain and transfer to a bowl of ice water to halt cooking. Drain again and set aside.
• For frozen beans: Thaw, drain, and press out extra moisture for best results.

To make the casserole:

• Melt the butter over medium-high heat in a large, oven-safe skillet (10 to 12 inches).
• Add the mushrooms, 1 teaspoon salt, and pepper.
• Cook, stirring occasionally, until the mushrooms release their juices, about 5 minutes.
• Stir in the garlic and cook for another 2 minutes.
• Sprinkle the flour over the mixture, stirring to coat the vegetables and allow the flour to absorb any liquid. As you cook, the flour will lightly brown at the bottom of the skillet, that’s what you want.
• Pour in the chicken broth, scraping up the browned bits with a spatula. Let it simmer for 3 minutes.
• Reduce the heat to low, add the half-and-half, and cook, stirring occasionally, until thickened, about 10 minutes, or longer for a thicker consistency.
• Remove from heat and mix in about a quarter of the shallots (or onions) along with all the green beans, stirring well to coat.
• Optional: transfer to a casserole dish.
• Sprinkle the remaining onions on top, then bake until bubbly, roughly 15 minutes.
• Remove from the oven and serve warm.