Blend the dried cep mushrooms in a food processor until finely ground, then combine them with the black pepper and herbs de Provence.
Cut the duck breasts lengthways into 3mm/1/8in thick slices.
Cut three of the young courgettes into thin slices using a vegetable peeler.
Assemble the tournedos by dividing the duck slices into six portions. For each portion, lay two slices of duck breast alongside each other, and stack the rest of the slices for that portion on top, alternating layers of fat and lean meat.
Divide the courgette strips into six portions. For each portion, lay the strips of courgette on top of the duck, season with the mushroom powder and season with salt.
Roll up the duck slices and courgette strips tightly to make tournedos and tie with string, to keep the cylinder shape in place.
Make six tournedos in this way.
Cook gently on the barbecue, or griddle pan, for five minutes each side.
Finely dice the remaining courgettes and toss with the onion, mint, lemon juice and olive oil.
To serve, cut the tournedos into thick slices and place on serving plates with the salad.