Heat 2 tablespoons of the oil in a saucepan, add the onion, garlic and rosemary and sauté gently for 5 minutes.
Season the flour with salt and pepper. Dip the tuna into the flour to coat evenly.
Heat the remaining oil in a frying pan, add the tuna and cook for 2–3 minutes or until golden. Turn over and cook on the other side for a further 1–3 minutes or longer, according to taste. Transfer to a dish lined with kitchen paper and keep warm in the oven.
Add the sun-dried tomatoes to the sautéed onions and stir well. Turn up the heat to high, add the wine, capers, olives and parsley and season with salt and pepper. Simmer for 2 minutes. Serve the sauce with the tuna steaks, lemon wedges and crusty bread.