Preheat the oven to 220°C 200°C fan
Place the mutton rump on a medium sized roasting tray. Trickle over the olive oil and season well with salt and pepper. Sprinkle over the chilli flakes and crushed toasted fennel seeds. Rub this mixture all over the mutton, making sure it gets into all the nooks and crannies.
Place the mutton in the preheated oven and roast for 15 to 20 minutes.
Meanwhile, half the tomatoes or quarter them if they’re big. Take the mutton out of the oven and rearrange the tomatoes around the meat. Season them lightly and scatter over the rosemary.
Return the mutton and tomatoes to the oven and continue to cook for a further 15 to 20 minutes or until the mutton is done to your liking. We like the rump medium rare, so we take the internal temperature up to about 65°C or there about.
While the lambs resting roughly chop the anchovies and the capers and stir them into the mayonnaise along with chopped parsley leaves.
Warm the focaccia in the oven for 5 minutes then split the bread. Spread each cut half with the anchovy and caper mayonnaise. Slice the Mutton into ½ cm thick slices and arrange these over the base of one of the halves.
Place the soft sweet tomatoes over the meat and pour over any rich tomatoey roasting juices from the tray. Scatter over the rocket and put the lid on. Cut the sandwich up into smaller servings and devour.