Peel the onions and finely chop them. Roughly chop the olives and the sundried tomatoes. In a frying pan, soften the onions in the olive oil, add the olives, garlic, thyme leaves and stew together. Combine this mixture with the sausage mince and season. Shape into sausages with your hands and roll them in the caul fat. Refrigerate for 24 hours before cooking.