Step 1: Preheat the Oven

Preheat your oven to 450°F (232°C).

Line a large baking sheet with parchment paper or a silicone baking mat. Set it aside while you prepare the onion topping.

Step 2: Prepare the Crispy Baked Onions

Separate the thinly sliced onion into individual pieces.

Prepare three shallow bowls. Place the flour in the first bowl, combine the panko breadcrumbs with the salt and black pepper in the second, and whisk together the egg and milk in the third.

Dip each onion slice into the flour, followed by the egg mixture, and finally the seasoned breadcrumbs.

Arrange the coated onion slices in a single layer on the prepared baking sheet.

Bake for approximately 20 minutes, turning the onions twice during cooking to encourage even browning.

Once the onions are golden and crispy, remove them from the oven and set aside.

Step 3: Blanch the Green Beans

Bring approximately one gallon of water to a rolling boil in a large saucepan.

Add 1 tablespoon of salt, followed by the trimmed green beans.

Cook for 5 minutes, then immediately transfer the beans to a large bowl filled with ice water.

Allow them to cool completely before draining thoroughly.

This process helps preserve their fresh color and keeps them from becoming too soft during baking.

Step 4: Cook the Mushrooms and Garlic

Melt the butter in a large, 10- to 12-inch oven-safe skillet over medium-high heat.

Add the sliced mushrooms, 1 teaspoon of salt, and 1/2 teaspoon of black pepper.

Cook for approximately 5 minutes, stirring occasionally, until the mushrooms release their moisture and begin to soften.

Add the minced garlic and continue cooking for another 2 minutes, stirring to prevent burning.

Step 5: Make the Creamy Mushroom Sauce

Sprinkle the flour over the mushroom mixture and stir until evenly combined.

Gradually pour in the chicken or vegetable broth, stirring continuously to prevent lumps.

Allow the mixture to simmer for approximately 3 minutes.

Reduce the heat to medium-low and slowly add the half-and-half.

Continue cooking for about 10 minutes, stirring occasionally, until the sauce becomes thick, smooth, and creamy.

Step 6: Assemble the Casserole

Remove the skillet from the heat.

Add the blanched green beans and approximately one-quarter of the baked onions.

Gently fold everything together until the vegetables are evenly coated in the mushroom sauce.

If your skillet is not oven-safe, transfer the mixture to a lightly greased 2- to 3-quart casserole dish.

Spread the remaining crispy onions evenly over the top.

Step 7: Bake Until Golden and Bubbly

Reduce the oven temperature to 400°F (204°C).

Bake the Green Bean Casserole from Scratch for approximately 15 minutes, or until the sauce bubbles around the edges and the onion topping is golden.

Remove from the oven and allow the casserole to rest for a few minutes before serving.

Serve warm and enjoy the combination of creamy vegetables and crunchy homemade onions.