Bring a large pot of water to a boil. Add beans and blanch for 2-3 minutes, until beans are tender. Transfer beans to a bowl of ice water. Drain well and spread into a 2.5 quart greased casserole dish.
Melt butter over medium-high heat in a skillet or frying pan. Add mushrooms and cook until tender and golden, about 7-8 minutes. Add garlic and cook 1 minute more. Remove from the pan and set aside.
Add chicken broth, cream, thyme leaves, and Dijon to the pan. Bring to a boil, scraping any bits off the bottom. Reduce heat to medium and simmer until thickened, about 10 minutes.
Stir in mushrooms with any juices. Cook for 2 minutes more. Taste and season with salt and pepper as needed.
Pour mushroom sauce over the green beans, stir, and bake for 15-20 minutes or until heated through.
Meanwhile, heat olive oil in a large skillet over medium-high heat. Add onions and cook for about 5-7 minutes, stirring often, until onions are browned and starting to crisp. Place onions onto a paper towel-lined plate to drain. Season with salt.
Top baked green beans with the fried onions and serve immediately.
Notes
Dry green beans well to keep them crisp and prevent a watery sauce.
Simmer sauce low and slow, whisking often for a smooth texture.
Prep Thanksgiving Green Beans a day in advance; refrigerate and keep the onion topping separate until baking.