Preheat your oven to 375°F (190°C). Bring a large pot of generously salted water to a boil. Add the trimmed green beans and blanch for 3–4 minutes until bright green and just barely tender.

Immediately transfer the green beans to a bowl of ice water to stop the cooking. Let them sit for 2 minutes, then drain thoroughly and pat dry. Set aside.

In a large oven-safe skillet or saucepan, melt the butter over medium-high heat. Add the sliced mushrooms in a single layer and cook undisturbed for 3–4 minutes until they begin to brown. Stir and continue cooking another 2–3 minutes.

Add the minced garlic and thyme and cook, stirring, for 1 minute until fragrant.

Sprinkle the flour over the mushroom mixture and stir to coat. Cook for 1 minute to eliminate the raw flour taste.

Slowly pour in the broth while whisking constantly to avoid lumps. Then add the milk and whisk until smooth. Bring to a simmer over medium heat, whisking frequently, until the sauce thickens to a creamy, coating consistency, about 4–5 minutes.

Stir in the parmesan (or nutritional yeast), soy sauce, salt, and pepper. Taste and adjust seasoning. Remove from heat.

Add the blanched, dried green beans to the sauce and stir gently to coat every bean thoroughly in the mushroom cream sauce.

If using an oven-safe skillet, spread the mixture evenly. If not, transfer to a greased 9x13-inch baking dish.

Spread the crispy fried shallots or onions evenly over the top of the casserole.

Bake uncovered at 375°F for 25–30 minutes until the sauce is bubbling at the edges and the topping is deep golden brown. Do not overbake.

Let the casserole rest for 5 minutes before serving. Enjoy hot.