Preheat your oven to 375°F (190°C) and coat a 9x13 casserole dish with cooking spray.

In a medium-sized bowl, mix the panko breadcrumbs, grated Parmesan cheese, minced garlic, and olive oil until evenly coated.

Fill a large pot with water and bring to a boil. Blanch the green beans for 3 minutes, then plunge into an ice bath.

In a large skillet, melt 2 tablespoons of butter over medium heat and sauté the sliced mushrooms for about 5 minutes.

Remove mushrooms, add remaining butter, and sauté onions for 5 minutes until translucent.

Return mushrooms to skillet, add thyme, salt, pepper, and flour and stir well. Gradually pour in milk while stirring until thickened.

Layer half of the blanched green beans in the casserole dish, pour half the sauce, add the remaining beans, and pour the rest of the sauce over them.

Sprinkle the panko topping evenly over the casserole and bake for 25 minutes until golden brown.

Let the casserole rest for 10 minutes before serving.