Boil the eggs in about 8 minutes until they are waxy. Quench and let them cool down. Drain tuna and pluck finely. Wash and finely chop the oregano. Peel onion and chop finely.
Add both to the tuna fish. Add mayonnaise, crème fraîche and lemon juice, stir in. Season cream with salt and pepper. Wash and slice the cucumber. Remove the crusts from the toast. Peel and slice the eggs.
Spread the tuna cream on 8 slices of toast. Cover 4 tuna toasts with cucumber and 4 more with egg. Place one tuna and egg toast on each slice with a tuna and cucumber toast and one uncoated toast.
Combine the rest of the toasts to make Tramezzini. Cut the Tramezzini diagonally in half and serve. Garnish with salad leaves.