Preheat the oven to fan 200C/ conventional 220C/gas 7. Lay the slices of bread on a baking sheet and bake for 8-10 minutes until toasted. Meanwhile, rub olive oil all over the racks of lamb, season generously with salt and pepper, then stand them in a roasting tin.
Take the bread out of the oven and put the lamb in. Roast the lamb for 10 minutes, then set aside for about 5 minutes to cool. While you’re waiting, break the toasted bread into a food processor, add the garlic, parsley and mint and season with salt and pepper. Chop until you have coarse crumbs, then tip into a wide, shallow dish.
When the lamb is cool enough to handle, spread mustard on the meaty sides, bottoms and ends. Hold a rack by the bones and sit it in the herby crumbs, then scoop up handfuls of crumbs and press them over the mustard. Repeat with the other 2 racks (you may have some crumbs left over, but you need this many to be able to scoop them up).
Roast the 3 racks of lamb, meaty side uppermost, for 20 minutes. If the crumbs begin to scorch, cover loosely with foil. Remove from the oven and leave to cool.
For a picnic, roast the lamb up to 6 hours ahead. When cold, separate into cutlets (this is simplicity itself, as the bones provide guidelines) and carefully wrap them in foil. Don’t forget to take jars of mustard and mint jelly to serve with the lamb.