Shell and devein the shrimp, rinsing only if necessary. Reserve the shells to enhance the broth.

Heat the other shellfish, one kind at a time, in the wine in a tightly covered pan. Remove them as they open to a dish.

Strain the shellfish liquor into a pot through a sieve lined with rinsed white paper toweling or tripled cheesecloth. Add the shrimp shells, about 2 cups of water, three or four dill sprigs, the coarsely chopped onion, bay leaf, and fish broth or clam juice. Simmer about 15 minutes, then strain and measure the broth. You should have at least 2 quarts; add wine or water if necessary to equal this amount. Add salt and pepper to taste. Heat the broth to just below a simmer and keep warm.

Meanwhile, saute the shrimp in 1 tablespoon of olive oil for about 1 1/2 minutes, or until just pink. Transfer the shrimp to the dish with the other shellfish.

Place the remaining olive oil and butter in a large casserole that can be used on the stove, cover, and sweat the diced onion and garlic over medium-low heat. When the onion is softened, add the rice, stir well, and reduce the heat. Saute the rice gently for about 10 minutes, then stir in about 2 cups of broth. Raise the heat again to medium-low. Continue stirring the risotto and adding more broth, about 1/2 cup at a time, as soon as the previous addition of liquid has been absorbed.

Meanwhile, remove the leaves from five or six dill sprigs and chop them. The risotto will be creamy and the rice al dente when it is done, in 16 to 20 minutes. Two or three minutes before this point, stir in the bivalves, then the shrimp, chopped dill, and green onions if desired. When the dish is heated through, remove it from the heat and add salt and pepper. If the risotto is too thick, add more hot broth.

Serve immediately. Pass lemon wedges and cheese if desired.