1 tablespoon butter 1/2 onion, chopped 1/4 cup melted butter 1/4 cup flour 2 cups milk 2 cups chicken stock 1 1/2 cups coarsely chopped broccoli florets 1 cup matchstick-cut carrots 1 stalk celery, thinly sliced 2 1/2 cups shredded sharp Cheddar cheese salt and ground black pepper to taste

Directions:

Melt 1 tablespoon butter in a skillet over medium-high heat. Saute onion in hot butter until translucent, about 5 minutes. Set aside.
Whisk 1/4 cup melted butter and flour together in a large saucepan over medium-low heat; cook until flour loses it's granular texture, adding 1 to 2 tablespoons of milk if necessary to keep the flour from burning, 3 to 4 minutes.
Gradually pour milk into flour mixture while whisking constantly. Stir chicken stock into milk mixture. Bring to a simmer; cook until flour taste is gone and mixture is thickened, about 20 minutes. Add broccoli, carrots, sauteed onion, and celery; simmer until vegetables are tender, about 20 minutes.
Stir Cheddar cheese into vegetable mixture until cheese melts. Season with salt and pepper to tast