Preheat the oven to 450°F (232°C). Lightly grease a 9x13-inch baking dish.
In a large skillet over medium heat, 1/2 pound bacon (diced) until crispy, about 12 minutes. Do not drain the grease.
Sprinkle 2 tablespoons all-purpose flour over the bacon and stir until the flour is fully absorbed, about 3 minutes. Add 2 cloves minced garlic and cook until fragrant, about 1 minute.
Pour in 1 cup chicken broth, 1 cup half and half, 1/2 teaspoon black pepper, and 1/4 teaspoon ground nutmeg. Bring the mixture to a boil, then reduce the heat and let it simmer until thickened, about 3 minutes.
Place 3 (15 ounce cans) green beans (drained) in the prepared baking dish. Pour the bacon mixture over the green beans, tossing to coat evenly.
Bake in the preheated oven for 15 minutes.
Remove the casserole from the oven and sprinkle 1 1/2 cups crispy fried onions over the top. Return to the oven and bake for an additional 5 minutes, until the onions are golden and crispy.
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Notes
To use fresh green beans instead of canned, start by trimming about 1 1/2 pounds of green beans and rinsing well. Set up a steamer basket in a large pot with about 1 inch of water beneath it. Bring the water to a boil, add the green beans to the basket, cover, and steam for 15 to 20 minutes, until tender but still crisp. Drain and let the beans cool slightly before adding them to the baking dish.