Cook the beans for 2 minutes in boiling salted water, drain and refresh in cold water. Remove the tough outer skins.
Cook the lasagne, 2 sheets at a time, for 2 minutes in a big pan of boiling salted water. Drain and drizzle with olive oil.
Meanwhile, heat the oil in a pan and cook the garlic for
30 seconds. Add the prawns, crème fraîche and zest and cook for 2-3 minutes until the prawns are cooked. Take off the heat.
Stir in the beans, Parmesan, lemon juice and mint, then season to taste.