Preheat the oven to 220°C/200°C fan/gas mark 7. Remove the lamb steaks from your fridge to allow them to come up to room temperature and season with salt and pepper.
Pick the rosemary leaves from their stalks and roughly chop (discard the stalks), then pop half into small bowl.
Roughly chop the walnuts and and pop into the rosemary bowl. Stir together and set aside.
Peel and grate the garlic (or use a garlic press). Trim and halve any large carrots (no need to peel).

Bring a large saucepan of water with 1/2 tsp salt to the boil. Peel and chop the potatoes into 2cm chunks.
When boiling, add the potatoes to the water and cook until you can easily slip a knife through, 15-20 mins.
Meanwhile, pop the carrots onto a large baking tray. Drizzle with oil, season with salt, pepper and the remaining rosemary, then toss to coat. Spread out in a single layer.
When the oven is hot, roast on the top shelf until tender, 20-25 mins. Turn halfway through.

Meanwhile, heat a drizzle of oil in a large frying pan on medium-high heat. When hot, carefully lay the lamb steaks into the pan. Fry until starting to brown, 2 mins each side.
Once seared, remove from the pan, then spread the mustard over 1 side of each. Sprinkle the rosemary walnut crumb over the mustard, pressing down so it sticks.
Pop the steaks onto the carrot tray and roast on the top shelf of your oven for 5 mins for medium-rare. TIP: Cook for 2-3 mins extra if you prefer yours more well done. IMPORTANT: Wash your hands and equipment after handling raw meat. The lamb is cooked when browned on the outside.

While the lamb cooks, drain the cooked potatoes in a colander and return to the pan, off the heat. Mash until smooth.
Season with salt and pepper, then stir in half the creme fraiche. Cover with a lid to keep warm.
When the lamb is cooked, transfer to a plate and leave to rest, covered loosely with foil.

Pop the (now empty) frying pan with any lamb juices on high heat (no oil).
When hot, add the garlic and spinach. Season with salt and pepper, then stir-fry until wilted, 2-3 mins.
Add the remaining creme fraiche and bring to the boil, stirring continuously, until thickened and piping hot, 4-5 mins. Taste and season with salt and pepper if needed. Set aside.

Share the mash between your plates with the lamb steaks on top.
Serve with the creamed spinach and roasted carrots alongside. Enjoy!