1-First Step: Begin by preheating your oven to 400°F. Line a large baking sheet with parchment paper and spray it with nonstick spray to prevent sticking. In a large bowl, toss the thinly sliced yellow onions with 1 tablespoon of olive oil until evenly coated.

2-Second Step: Add 1/2 cup all-purpose flour (or alternative flour), 1/2 cup panko breadcrumbs (or gluten-free option), 1/2 teaspoon garlic powder, and 1/2 teaspoon salt to the bowl with the onions. Toss everything together until onions are thoroughly coated with the mixture.

3-Third Step: Spread the coated onions in a single layer on the prepared baking sheet. Bake for 15-25 minutes, tossing every 8-10 minutes, until the onions are golden and crispy. Watch carefully to prevent burning. Once done, set aside and reduce oven heat to 350°F if using the primary baking method.

4-Fourth Step: Bring a large pot of salted water to a rolling boil. Add the trimmed green beans in batches (up to 2.5 pounds at once) and blanch for just 2-3 minutes until bright green but still crisp-tender.

5-Fifth Step: Remove the beans with tongs and immediately place them in an ice bath to stop the cooking process and preserve their vibrant color. Drain thoroughly and pat dry with paper towels or a clean kitchen towel.

6-Sixth Step: In a large skillet over medium-high heat, melt 2 tablespoons of butter (or vegan alternative). Add the sliced baby bella mushrooms, seasoning with salt and pepper to taste. Cook, stirring occasionally, for 8-10 minutes until the mushrooms release their liquid, then reabsorb it and turn golden brown.

7-Seventh Step: Stir in 1 tablespoon fresh thyme leaves, 1/2 teaspoon red pepper flakes, and 3 cloves of minced garlic. Cook for about 1 minute until fragrant, being careful not to burn the garlic.

8-Eighth Step: Using the same skillet, melt 3 tablespoons butter over medium heat. Whisk in 1/4 cup all-purpose flour and cook for just 15-30 seconds to form a paste (roux), whisking constantly to prevent burning.

9-Ninth Step: Slowly whisk in 2 cups of low sodium chicken broth (or vegetable broth), then gradually add the remaining 1/4 cup, ensuring no lumps form. Increase heat to bring the mixture to a boil, then reduce heat and simmer for 5 minutes, stirring occasionally, until the sauce thickens like gravy. If too thick, add more broth 1/4 cup at a time.

10-Tenth Step: Remove from heat. Stir in any minced garlic that wasn’t added with the mushrooms, then add 1/2 cup grated parmesan cheese until melted and incorporated. Season with salt and pepper to taste.

11-Eleventh Step: Heat 1 tablespoon olive oil in a saucepan over medium heat. Cook sliced onion and mushrooms until soft and golden, which should take about 15 minutes.

12-Twelfth Step: Add the remaining 1 tablespoon oil and 1/4 cup flour; stir until the flour turns golden. Gradually whisk in 2 cups of milk, then add 1 teaspoon salt, 1/2 teaspoon black pepper, and 1/8 teaspoon nutmeg.

13-Thirteenth Step: Cook, stirring, until the sauce thickens (8-10 minutes). Remove from heat and stir in 7 ounces of Greek yogurt (or non-dairy alternative) until fully incorporated.

14-Fourteenth Step: Spray a 2-quart baking dish or 9×13-inch pan with nonstick spray. Add the blanched green beans, cooked mushrooms, and 1/4 of the crispy onions; toss gently to combine.

15-Fifteenth Step: Pour the primary sauce over the mixture. Cover tightly with foil. Bake at 350°F for 25-30 minutes.

16-Final Step: Remove foil and sprinkle the remaining crispy onions on top. Bake uncovered for 5-10 minutes more until onions are crisp and casserole is bubbly. Serve immediately while hot and fresh.

17-Alternative Method: Preheat your broiler. Coat a 2-quart broiler-safe baking dish with nonstick spray. In a small bowl, mix 1/2 cup whole wheat panko (or GF breadcrumbs), 1/4 cup grated Parmesan (or nutritional yeast + 1/2 tbsp lemon juice for vegan), and 2 tablespoons chopped fresh parsley (set extra for garnish); stir in reserved 2 tablespoons olive oil for topping.

Layer half the green beans and half the alternative mushroom/yogurt sauce in the dish; repeat layers. Top evenly with breadcrumb mixture. Broil 1-3 inches from heat until bubbly and topping is browned (1-3 minutes). Let stand 10 minutes before serving. Garnish with fresh parsley.