Preheat the oven to 220C, fan-forced. Remove 30g butter from the fridge to soften. Put the potatoes in a medium saucepan, cover with cold salted water and bring to the boil. Reduce the heat to medium and cook for 12-15 mins until tender. Drain. Reserve the pan.
Step 2 — Prep ingredients
Meanwhile, trim the stems from the sugar snap peas, removing the strings from one side, then halve on an angle. Crush or finely chop garlic. Finely chop rosemary and tarragon leaves, discarding stems. Combine the softened butter, garlic and half the rosemary in a bowl. Combine the remaining rosemary and 2 tsp sea salt flakes (see cooking tip) in a separate bowl.
Step 3 — Roast potatoes
Line 2 oven trays with baking paper. Put the potatoes on a lined tray and lightly smash with a fork. Drizzle with 1 tbs olive oil and season with half the rosemary salt. Roast for 20-25 mins until golden and crisp.
Step 4 — Prep and roast lamb
While potatoes are roasting, drizzle lamb with 2 tsp olive oil and season with the remaining rosemary salt and pepper. Heat a medium frypan over high heat. Cook lamb, turning regularly, for 4 mins or until browned. Transfer to remaining lined tray. Roast for 3-5 mins for medium-rare, or until cooked to your liking. Remove from oven, dot with rosemary butter and rest for 8 mins.
Step 5 — Cook peas
Bring water to the boil in the reserved pan. Add the peas and sugar snap peas, return to the boil and cook for 1 min. Drain.
Step 6 — Dress pea salad and serve
Put the peas, vinaigrette and tarragon in a bowl, season with salt and pepper and toss to combine. Slice the lamb. Divide the lamb, potatoes and pea salad among plates. Drizzle the lamb with the buttery tray juices to serve.