Photograph: Brett Stevens
Preheat the oven to 150°C, fan 140°C, gas 2. Heat ½ tablespoon of the oil in a large frying pan over a high heat. Season and sear the joints one at a time on all sides and both ends, for 30-60 seconds each side until well-coloured. Place them fat-side up on a board and brush the mustard over the top and down the sides. Scatter over the thyme and gently press into the mustard.
Place the joints in a large roasting tin, drizzle over the remaining oil and roast for 1 hour 10 minutes for rare, then leave to cool. Slice and arrange on serving plates on a bed of rocket.
For the garlic soured cream, blend the soured cream, garlic, lemon juice and salt in a bowl, then spoon into a couple of serving bowls. Cover and chill until required. To serve, season with freshly ground black pepper or cayenne pepper, if you prefer.
Kitchen secret: roasting sirloin in a slow oven makes for superbly tender eating, and leaves it really rare in the centre.