Wash the cherry tomatoes and cut into eighths. Peel and finely chop the garlic.
Juice the lemon halves. Cut the olives into slices.
Wash the rosemary, shake dry and chop the leaves.
Drain the capers and mix with the garlic, olives, lemon juice, rosemary, harissa and olive oil. Add 1 tbsp chopped onion.
Mix with the tomatoes and season with salt and pepper.
Halve the feta cheese and place each piece in the middle of a large sheet of foil (approx. 30 x 30cm). Spread the tomato-caper mixture over the cheese.
Fold the foil into packets and place in a casserole dish. Bake in a preheated oven at 180°C (160°C fan, gas 4) for 15 minutes. Place the packets on plates, open carefully and serve hot.