If using fresh cockles, rinse them thoroughly, and put in a saucepan with the water. Steam for 10 minutes or until the cockles are all open, discarding any that remain closed. Put a fine sieve over a bowl, and pour the cockles into it. Reserve the liquor. Remove and discard the shells when cool enough to handle. Rinse the shelled cockles to remove any grit.
Roll out the pastry, and use to line a 9–10 inch / 23–25.5 cm. pie or quiche dish. (There will be pastry left over for the lattice top.) Wash and slice the spring onions or leeks. Remove and discard the bacon rind, and cut the bacon into matchsticks. Fry it gently for a few minutes, and then drain. Layer the cockles, spring onions and bacon in the pie dish. Grind on a little pepper. Beat the cream and eggs together with a little cooking liquor if you have cooked the shellfish from scratch, or 2–3 tablespoons water if not. Pour the custard over the filling. Roll out the remaining pastry, cut strips and use to make a lattice top. Bake in a preheated oven at 200°C / 400°F / Mark 6 for about 15 minutes then turn down the heat to 180°C / 350°F / Mark 4, and cook for a further 15–25 minutes.