Mix flour, salt, pepper, paprika, garlic powder, and onion powder in a shallow dish.

Drain onions, dredge in flour mixture, and shake off excess.

Preheat air fryer to 400°F. Spray basket with oil, add onions in a single layer, and spray the tops.

Air fry 7–10 minutes, shaking halfway, until golden. Transfer to a wire rack to cool and stay crispy.

Soak onions in buttermilk 20–30 minutes.

Whisk flour, cornstarch, salt, pepper, garlic powder, and paprika in a dish.

Heat oil in a skillet or Dutch oven to 350°F.

Dredge onions in flour mixture, then fry in batches for 2–3 minutes until golden.

Drain on paper towels and season lightly with salt.

Preheat oven to 350°F.

Bring a large pot of salted water to a boil.

Cook beans for 4 minutes, then transfer immediately to an ice bath to stop cooking. Drain and set aside.

Melt butter in a large pan over medium heat.

Add mushrooms and cook 5–6 minutes until softened and liquid has evaporated.

Stir in garlic and cook 30 seconds.

Sprinkle flour, mustard powder, salt, and pepper over mushrooms. Whisk and cook for 1 minute.

Slowly whisk in milk or cream. Bring to a boil, stirring often, until thickened (about 5 minutes).

Remove from heat and stir in Gruyère and Parmesan until melted.

Stir green beans and half the crispy onions into the mushroom sauce.

Spray a 9×13 baking dish with nonstick spray and pour in the mixture.

Cover with foil and bake 30 minutes, until bubbly.

Uncover, top with remaining onions, and bake 5 more minutes.