Blanch the Green Beans

Bring a large pot of salted water to a boil. Add the green beans and cook about 5 minutes, until bright green and crisp-tender. Transfer them to an ice-water bath to stop the cooking and keep their texture.

Build the Mushroom Base

Warm the butter or oil in a large oven-safe skillet over medium heat. Add the shallot and garlic with a pinch of salt and pepper. Cook 2–3 minutes until fragrant.

Add the mushrooms and cook 3–4 minutes more, until lightly browned and softened.

Make the Sauce

Sprinkle the flour over the veggies and stir to coat. Cook for 1 minute.

Slowly whisk in the vegetable broth, then the milk. Keep whisking until it starts to bubble and thicken.

Lower the heat and let it simmer 5–7 minutes. Taste and season with more salt and pepper if needed. It should resemble a thick, velvety cream of mushroom soup.

Combine Everything

Remove the skillet from the heat. Stir in about one-third of the crispy onions so they melt into the sauce.

Add the green beans and gently toss until everything is well coated.

Bake

Top with the remaining crispy onions.

Bake at 400°F for 15 minutes, or until hot, bubbly, and lightly golden on top. Serve while the onions are still crisp.