Ingredients: 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 …