Scotch Lamb Neck Fillet with Orange, Feta and Olives

4 Scotch Lamb neck fillets, trimmed

1 onion, peeled and sliced

2 garlic cloves, peeled and crushed

10 new potatoes halved

1 orange, zest removed and halved

1 tin good quality chopped tomatoes

250 ml lamb or chicken stock

200g feta cheese

50g black olives, stoned

olive oil

sea salt and black pepper