Melt the butter in a large flameproof casserole and add the onion and mint. Fry gently for about 20 mins until the onion is soft but not browned.
Add the peas and salt, then cover and continue cooking until the peas are tender. This will take 5 mins for frozen and 15 mins for fresh.
Meanwhile bring the wine to a boil in a large saucepan. Add the clams and steam for 5-10 mins until opened. Using a slotted spoon add the clams to the peas, discarding any that have not opened.
Strain the juice from the clams into the stew through a fine sieve.