Heat a large pot of water to boiling. Add green beans and cook about 5 minutes. Drain and set aside. Preheat oven to 350 degrees.
Wipe out the pot and melt butter over medium-low heat. Add flour, salt, pepper, sage, and thyme and whisk about 1 minute.
Add milk and vegetable broth and whisk to combine. Increase heat to medium and whisk occasionally until sauce thickens. (It’s ready when the whisk leaves a defined trail.) Remove from heat and stir in green beans.
Spray an 11 x 7-inch baking dish with cooking spray. Pour in green bean mixture and smooth. Sprinkle with fried onions.