Preheat oven to 450 degrees F. Lightly grease a baking sheet. In a large mixing bowl, toss sliced onions together with 1/4 cup flour, panko bread crumbs, and salt. Spread out in an even layer on the baking sheet. Bake in the preheated oven for 30 minutes, tossing every 10 minutes.
Meanwhile, cook the bacon in a skillet over medium heat until crispy. Do not drain the grease. Add in garlic and saute 30 seconds. Stir in flour until completely dissolved. Pour in chicken broth, half and half, black pepper, and nutmeg. Bring to a boil and let simmer until thickened, about 3 minutes.
Defrost your green beans in the microwave, or steam if using fresh. Place green beans into a lightly greased 9x13 pan. Pour bacon mixture over the green beans and toss to coat. Bake in the 450 degree oven for 15 minutes.
Remove from the oven and sprinkle the cooked, crisp onions on top. Return to the oven for 5 minutes before serving.