Peel the onions and cut into quarters or eighths, depending on the size. Skin, eighth and core the tomatoes.
Heat the butter in a pan and sauté the onions in it for about 10 minutes. Add tomatoes, broth and wine. Drizzle the fish with lemon juice, season with salt and pepper. Place in the pan on top of the vegetables. Add the tarragon, cover and cook for about 10 minutes. Salt and pepper.