Chop onion fine, place in a frying pan with olive oil and simmer until it is of pale pink color. Add the washed rice, seasoning and water and cook for about 10 minutes. Place mussels in a sieve and run cold water over them. Take out any black spots you might see. Place midia on top of the onions, add the currants and nuts. Cover and simmer on very low flame until all the rice is cooked. Serve this cold. Garnish it on a bed of green lettuce, or fill the mixture into oyster shells. (When mussels are not in season, a satisfactory Midia appetizer can be made by using canned mussels).