Lamb meatballs with rosemary tomato sauce

INGREDIENTS

• 2 tsp olive oil
• 1 pack 12 ready-made lamb meatballs (approx 450g)
• 4 onions , finely sliced
• 4 garlic cloves , finely sliced
• 1 tbsp finely chopped rosemary leaves, plus extra to sprinkle
• 2 x 400g cans chopped tomatoes
• 400g pasta , cooked, to serve