Step 1: Prepare the green beans by snapping off the ends. Blanch them in boiling salted water for 3-5 minutes until bright green and crisp-tender. Immediately transfer to an ice bath to stop cooking, then drain well.
Step 2: In a large oven-safe skillet or Dutch oven, melt 4 tablespoons of butter over medium heat. Add the diced yellow onion and cook until softened, about 5 minutes.
Step 3: Add the sliced cremini mushrooms to the skillet and cook, stirring occasionally, until they release their liquid and begin to brown, about 8-10 minutes.
Step 4: Stir in the minced garlic, fresh thyme leaves, and fresh sage leaves. Cook for 1 minute until fragrant.
Step 5: Sprinkle 1/4 cup of all-purpose flour over the mushroom mixture and cook, stirring constantly, for 1-2 minutes to create a roux.
Step 6: Gradually whisk in the whole milk and chicken broth, stirring constantly to prevent lumps. Continue whisking until the sauce thickens. Season with 1 teaspoon salt, 1/2 teaspoon black pepper, and a pinch of nutmeg. Reduce heat to low and simmer for 5-7 minutes, allowing the sauce to further thicken and flavors to meld.
Step 7: Gently fold the blanched green beans into the creamy mushroom sauce, ensuring they are thoroughly coated. If not using an oven-safe skillet, transfer the mixture to a 9×13 inch baking dish.
Step 8: To make the crispy onions: thinly slice the two large yellow onions. In a bowl, toss them with 1/4 cup of all-purpose flour and 1/2 teaspoon of salt until evenly coated.
Step 9: Heat 2-3 cups of vegetable oil in a deep skillet or pot to 350°F (175°C). Fry the coated onions in batches, being careful not to overcrowd the pan, until golden brown and crispy, about 3-5 minutes per batch. Remove with a slotted spoon and drain on a paper towel-lined plate. Season lightly with salt.
Step 10: Preheat your oven to 375°F (190°C). Bake the casserole, uncovered, for 20-25 minutes, or until the sauce is bubbly and heated through.
Step 11: Remove the casserole from the oven and generously scatter the homemade crispy fried onions (or store-bought) over the top. Serve immediately and enjoy!