Preheat the oven to 200°C.
Roast lamb trimmings in the oven for 15-20 minutes until golden.
Sauté onion and garlic in butter and 15ml oil for 5 minutes.
Stir in tomato purée, then add trimmings, tomatoes, herbs, and 400ml stock.
Simmer for 40 minutes, strain through a sieve, and keep warm.
Blend herbs, breadcrumbs, and 15ml oil into a paste.
Sear lamb racks in remaining oil for 5 minutes until browned.
Brush lamb with mustard and press on the herb crumb mixture.
Roast lamb at 200°C for 20 minutes for medium doneness.
Boil potatoes and garlic in 1.2l stock for 15 minutes.
Reduce 300ml of reserved potato stock by half, add cream, and reduce again.
Toss potatoes in the cream reduction to coat.
Boil green beans for 5 minutes and drain.
Carve lamb, serve with potatoes and beans, and drizzle with gravy.