In a large, heavy saucepan, over medium heat, cook 1 large onion, chopped in 3 tbsp unsalted butter until softened, about 5 minutes. Step 2 Add 1/4 cup all-purpose flour and stir until smooth. Whisk in 3 cups chicken broth and 2 cups whole milk, then add 1 large sweet potato, peeled and diced (1 1/2 cups),1 bay leaf, 1 tsp fresh thyme leaves, ½ teaspoon salt, and ¼ teaspoon pepper. Step 3 Bring to a boil, reduce heat, and simmer, stirring occasionally, until the potatoes are just tender, about 8 minutes. Step 4 Add 1 pound skinless salmon fillet, cut into 1-inch pieces and 1 small broccoli stalk, cut into pieces (about 1/2 cup)and simmer until the salmon is cooked through and the broccoli is tender, about 5 minutes more.