Peel the onions and slice them very thinly. Gently separate the rings, trying to keep as many intact as possible.

Set up three shallow bowls. Put the flour in the first. Whisk the egg and milk together in the second. Add the panko to the third and season it lightly with salt and black pepper.

Coat each onion ring in the flour and shake off the excess. Dip it into the egg mixture, then coat it well with panko.

Arrange the breaded onions on the prepared baking sheet in a single layer without overlapping. Lightly spray them with cooking spray or drizzle very lightly with oil.

Bake for about 15 minutes, turning them partway through, until golden and crisp. Keep a close eye on the thinner pieces near the end because they can brown quickly. Set aside.

Reduce the oven temperature to 350°F.

Trim the green beans. Bring a large pot of lightly salted water to a boil, then cook the beans for about 8–10 minutes, until tender but still firm.

Transfer the beans immediately to a bowl of ice water to stop the cooking. After a few minutes, drain them again and dry them VERY well with a clean kitchen towel or paper towels.

Melt the butter in a large skillet over medium heat. Add the garlic and cook for about 30 seconds.

Add the sliced mushrooms and cook for about 8 minutes, stirring occasionally, until they release most of their moisture and begin to brown.

Sprinkle the flour over the mushrooms and stir well. Cook for about 1 minute so the flour absorbs the cooking juices.

Gradually pour in the broth while stirring continuously until the mixture is smooth.

Add the milk and heavy cream. Reduce the heat slightly and cook for a few minutes, stirring often, until the sauce starts to thicken. It should be creamy but still a little loose because it will continue thickening in the oven.

Season with salt and black pepper as needed.

Add the well-dried green beans to the mushroom sauce and gently stir until evenly coated.

Roughly crumble about one-third of the crispy onions and fold them into the green bean mixture.

Transfer everything to a baking dish and spread it out evenly.

Bake at 350°F for about 15 minutes, until the sauce is hot and gently bubbling around the edges.

Remove the casserole from the oven and scatter the remaining crispy onions over the top.

Return it to the oven for another 5 minutes, just long enough to warm the onions without sacrificing all that crunch we worked unnecessarily hard to protect haha.

Let the casserole rest for about 5 minutes before serving.