01 -
Boil a large pot of salted water and blanch green beans until bright green and just tender, about 3–4 minutes. Immediately transfer to an ice bath to halt cooking and preserve color.
02 -
In a large skillet, melt butter over medium heat. Add mushrooms, garlic, and shallots, sautéing until veggies are soft and fragrant, approximately 5–7 minutes.
03 -
Sprinkle flour evenly over the mushroom mixture, stirring continuously to form a roux. Gradually whisk in milk and broth, cooking until sauce thickens to a creamy consistency, about 4–5 minutes.
04 -
Season sauce with salt, black pepper, and a pinch of nutmeg, adjusting to taste.
05 -
Fold blanched green beans into the sauce, ensuring even coating. Transfer mixture to a greased 22x22 cm (9x9 inch) baking dish.
06 -
Evenly spread French fried onions over the top of the green bean mixture.
07 -
Bake in a preheated oven at 190°C (375°F) for 20–25 minutes until sauce is bubbly and topping is golden and crisp.