Cook the peas in boiling water for about 4 minutes until soft. Drain and return to the saucepan with the butter and some salt and pepper. Use a potato masher to crush the peas so that they are almost puréed. Stir in the mint and set aside.
Mix the breadcrumbs with the lemon rind, some salt and pepper and the parsley, and spread on a large plate. Pour the beaten egg into a shallow bowl and dip the hake fillets into it before placing them on the breadcrumbs. Turn the fish in the breadcrumbs, making sure the flesh is completely covered.
Heat the oil in a frying pan over a medium-high heat and add the fish to the pan. Cook for about 4 minutes, turning once, until the fish is cooked through and the breadcrumbs are golden and crispy.
Spread the crushed peas over the bases of the rolls and lay the fish on top. Top with the lids, then toast in a sandwich grill for 2–3 minutes, or according to the manufacturer's instructions, until the bread is golden and crispy. Cut each sandwich into quarters and serve immediately with tartare sauce.