Preheat oven to 350F (175C).

Blanch the green beans. Bring a large pot of salted water to a rolling boil. Add green beans and boil for 5 minutes. Meanwhile, prepare an ice bath by filling a large bowl with ice water. Drain green beans then immediately submerge in the ice bath. Let sit while you prepare the sauce.

In a large skillet over medium heat, cook butter until melted.

Add onion and cook until softened and translucent.

Add garlic, and cook until fragrant, about 30 seconds.

Add mushrooms and cook, stirring frequently, until softened.

Sprinkle flour over the veggies and cook until absorbed, then cook an additional 30 seconds longer to lightly toast (builds flavor!).

While stirring, gradually drizzle in chicken broth, then add milk, cream, salt, black pepper, and thyme. Cook, stirring frequently, until sauce is thickened (should coat the back of a spoon).

Drain green beans and add to the sauce, stir until beans are covered with sauce.

Spread evenly into a 2qt baking dish and sprinkle the top with French fried onions.

Transfer to center rack of 350F (175C) oven and bake, uncovered, for 30 minutes (will likely be bubbling around the edges).