Preheat oven to 220°C. Combine mustard, lemon juice and garlic in a small bowl, season to taste and brush over lamb, then drizzle with a little olive oil and place in one side of a roasting pan. Roast while you prepare the potatoes.
Cook potatoes in a large saucepan of well-salted boiling water for about 2 minutes until just starting to soften, drain well, return to pan and toss with thyme, rind and about 1½ tbsp olive oil, season to taste with salt and freshly ground pepper. Spread out evenly in the other side of the roasting pan and roast until potatoes are crisp and lamb is cooked to your liking, tossing peas into pan in the last 5 minutes of cooking (55-60°C internal temperature or 20-25 minutes for medium-rare, 60-65°C or 25-30 minutes for medium), set aside to rest for 10 minutes.
For the mint dressing, coarsely crush mint, garlic, a good pinch of salt and plenty of freshly ground pepper in a mortar and pestle to release all the beautiful aromas. Stir in vinegar and about 2½ tbsp olive oil and serve spooned over sliced lamb with crisp potatoes and peas.
Bring the mini lamb roast out of the fridge for at least 30 minutes before cooking to bring it up to room temperature, otherwise you’ll need to allow more cooking at resting time.
To make this a meal to serve a bigger group, consider using the same technique and flavours with a lamb leg.