1 pound green beans, ends trimmed
1 tablespoon vegetable or canola oil
1/2 cup thinly sliced onion
8 ounces cremini mushrooms, rinsed and sliced
1 1/2 teaspoons kosher salt
1/8 teaspoon garlic powder
1/8 teaspoon freshly ground black pepper
1/2 cup no-sugar, low-sodium chicken broth
1 teaspoon chopped fresh thyme leaves, plus a few sprigs for garnish
1/2 cup sour cream
Preheat oven to 350 degrees F.
Bring a small pot of water to boil and season lightly with salt. Boil the green beans until tender, but still crispy, about 5 minutes. Drain and chill beans in an ice water bath or under running cold water. Drain again and transfer to a bowl.
Heat the oil in a large pan over medium-high heat. Add onion, mushrooms, salt, garlic, and pepper and cook, stirring, about 2 minutes. Add the chicken broth and thyme and cook until almost all the liquid is evaporated, about 3 to 4 minutes. Transfer the mushroom mixture to the bowl of green beans, add the sour cream, and toss to combine.
Pour the green bean mixture into the glass pie pan. Bake the casserole for 10 minutes, top with french onions and bake until onions are browned.