Step 1Melt the butter in a heavy, enameled cast iron saute pan (such as Le Creuset) over medium heat. Add the spring onions or leeks with a generous pinch of salt and a few grinds of pepper. Cook, stirring, 4 to 5 minutes. Add the vermouth and clam juice and bring to a high simmer.
Step 2Add the potatoes; season again. Cover and cook over low heat until the potatoes are fork tender, about 15 minutes.
Step 3Season the monkfish with salt and pepper and add to the pan. Cover partway and cook over low heat 7 minutes. Add the scallops and cook 1 minute, stirring constantly, until they are almost translucent. Add the crab, peas, cream and dill and cook 2 minutes, or until the crab is warmed through.
Step 4Using a slotted spoon, transfer the seafood and vegetables to a warm, shallow serving dish. Raise the heat and reduce the cooking liquid until it has the consistency of sauce. Taste and adjust the seasoning, adding more salt and pepper as needed. Pour over the seafood and vegetables.