Scatter the seafood pieces on top of vegetables, placing the clams/mussels closer to the bottom of the pan. Add 3 cups of broth and the seasoning paste, reserving 1 tablespoon to add as necessary to your liking. Top it with the watercress (or crown daisy), and cook over medium high heat until the clams/mussels are open, the fish is cooked through and the vegetables are softened. Drizzle the optional lemon juice over. Stir occasionally.