Rub the lamb with the oil, garlic and rosemary. Cover with clingfilm and refridgerate overnight.
Preheat the oven to 200C/180C Fan/Gas 6.
Season the rumps with salt and pepper. Add the butter to an ovenproof frying pan over a medium heat. Once hot, add the rumps and cook on each side until golden brown. Transfer to the oven and bake for 10 minutes, or until cooked to your liking. Leave to rest for 10 minutes (or the amount of time it was cooked for).
To make the pesto, toast the pine nuts on a baking tray in the oven until golden. Blend the peas with the garlic until they break down a little. Add the toasted pine nuts, Parmesan, oil garlic and mustard and blend until a coarse paste. Add more oil and blend again if needed.
To make the salad, whisk the vinegar, oil and mustard with some salt and pepper in a small bowl. Mix the radishes, lettuces, mint and fennel together in a large salad bowl. Just before serving, pour the dressing over the salad.
To serve, slice the lamb rumps and divide equally between four plates. Dress with the pesto and serve with the salad.