Preheat the oven to 220C, fan-forced. Line an oven tray with baking paper. Finely chop the parsley leaves, discarding the stems. Peel the potatoes and cut into 3cm chunks. Combine the barbecue seasoning and 2 tsp olive oil in a large bowl and season with salt and pepper. Add the lamb and turn to coat.
Step 2 — Mash potato
Put the potato in a medium saucepan of cold salted water and bring to the boil. Reduce the heat to medium and cook for 13-15 mins until tender. Drain and return to the pan. Mash the potato until smooth. Stir in the parsley and 1 tbs milk. Taste, then season with salt and pepper.
Step 3 — Roast lamb
While the potato is boiling, heat a medium frypan over high heat until hot. Cook the lamb, turning regularly, for 4 mins or until browned. Transfer to the lined tray, reserving the pan. Roast the lamb for 10-13 mins for medium-rare, or until cooked to your liking. Remove from the oven and rest for 8 mins.
Step 4 — Make gravy
Crumble the stock cubes into a heatproof jug, add 250ml (1 cup) boiling water and stir to dissolve. Melt 20g butter in the reserved pan over medium heat. Add 1 tbs flour and cook, stirring, for 2 mins or until foaming. Gradually add stock and any resting juices, stirring constantly to prevent lumps forming. Bring to a simmer and cook, stirring, for 3 mins or until thickened.
Step 5 — Serve up
Combine 2 tsp olive oil and 1 tsp balsamic vinegar in a large bowl and season with salt and pepper. Add the spinach and walnuts and toss to combine. Divide the lamb, herby mash and walnut salad among plates, drizzle over the gravy and enjoy!
Step 6 — Kitchen 101
The secret to smooth gravy? Add the stock gradually, stirring constantly, to achieve a creamy consistency before adding more stock. If lumps form, use a whisk and more elbow-grease, and whisk until smooth.