Best Green Bean Casserole

Preheat oven to 375F (unless you are making this in advance, then don't) and spray a 2 to 3-quart casserole dish or 9x13-inch pan with cooking spray; set aside.

Bring a large stockpot of water to a boil over high heat. After it's boiling, add the green beans and cook them for 3 to 4 minutes (this is known as blanching). Get a large bowl of ice and water waiting (ice bath).

Drain the beans with a colander and immediately transfer them into the ice bath. Let them rest there while you move on making the sauce.

To a large sauté pan, add 2 tablespoons butter, red onions, and sauté them over medium to medium-high heat for about 5 minutes, or until they begin to soften; stir frequently. Take care not to overcook them so they don't lose their pretty red color; set aside.

To a separate large sauté pan or Dutch oven, add the mushrooms, remaining 4 tablespoons butter, and sauté over medium-high for about 5 minutes, or until the mushrooms have softened; stir frequently.

In the final 1 minute of cooking, add the garlic, and cook until fragrant; stir constantly.

Even sprinkle the flour over the top, and cook for about 2 minutes to ensure you don't taste raw flour in the finished dish; stir constantly.

Slowly add the 2 cups of milk into the mushrooms and flour, and stir nearly constantly for about 3 to 5 minutes to create a thickened gravy texture; move patiently and slowly so that lumps are not created and the sauce stays smooth and creamy.

Add the chicken broth, thyme, salt, pepper, and stir frequently until the sauce thickens, about 3 to 5 minutes.

After it has thickened, add the shredded white cheddar cheese, and stir until the cheese has completely melted and the sauce is smooth and of uniform texture; take it off the heat and set it aside.

Take the green beans from the ice bath, transfer them into your casserole dish, and if you'd like blot them off with paper towels after they're in the baking dish.

Evenly drizzle the cheese sauce with the mushrooms over the green beans; smooth it as necessary with a spatula.

Evenly top with the red onions.

Evenly sprinkle the panko bread crumbs over the top.

Evenly sprinkle the Parmesan cheese over the top. Tip - If you're making this dish in advance, stop here. Cover tightly with foil and refrigerate for up to 24 hours. You could also freeze it for up to 3 months although finding that much freezer space for that long could be tricky. When you're ready to bake it, thaw at room temp on the counter and then bake as directed in step 16.

Bake for 20 minutes or until bread crumbs and Parmesan are lightly golden browned; start checking at 15 minutes and rotate pan midway through if your oven doesn't bake evenly so the topping doesn't burn. Tip - If you're baking this with other items in the oven that need a higher oven temp, that's fine. Cover it with foil and in the final 5 minutes of baking, take the foil off so the topping crisps up and the cheese lightly browns.

Evenly sprinkle with the parsley to garnish and serve. Recipe will keep airtight in the fridge for up to 1 week and in the freezer for up to 3-4 months, noting that in either situation the topping will lose its crispness and if freezing, the green beans will undergo a texture change and will be mushier, which is inevitable with freezing fruits or vegetables.