Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub and peel the potatoes. Chop them into small chunks, around 2cm across. Pop them in a bowl and toss with 2 tsp olive oil. Spread out in a roasting tin. Roast for 15 mins.
While the potatoes roast, add 1 tsp fennel seeds to a mortar and pestle and crush (or use a small bowl and the bottom of a jam jar). Finely chop the sage leaves. Add them to the fennel seeds with a pinch of salt and pepper. Stir together to form a dry rub.
Pop the pork tenderloin on a board and use a small, sharp knife to slash it all over. Rub the sage and fennel mixture all over the pork. Turn the oven down to 180°C/Fan 160°C/Gas 4. Take the roasting tin out of the oven, nestle the pork in the tin and return to the oven for 15 mins.
While the pork roasts, pull any ragged leaves off the sprouts. Halve, then finely slice them. Rinse, pat dry with kitchen paper and tip into a large bowl. Set aside.
Cut the apples into quarters and slice out the cores. Add them to the tin and roast for another 15 mins (so the pork roasts for 30 mins in total).
While the pork finishes roasting, cut the pomegranate in half. Holding one pomegranate half in the palm of your hand, cut-side down, whack it with a wooden spoon. The seeds should fall through your fingers. Catch them in the bowl with the Brussels sprouts. Halve the fennel bulb. Finely slice it and add to the bowl. Keep any frondy bits back for garnishing.
Whisk 1 tbsp balsamic vinegar with 1 tsp oil. Pour this over the sprout slaw. Season. Toss together. Serve the roast pork in slices with the slaw, roast potatoes and apples, garnished with fennel fronds.