Preheat the oven to 200°C, 400°F, gas mark 6.
Heat 1 tbsp of the oil in a pan and sauté the onion for 3-4 minutes. Stir in the fennel seeds then pour in the wine and simmer for a couple of minutes before adding the tomatoes. Simmer for 5 minutes.
Cook the potatoes in a pan of boiling water until tender. Pour the tomato sauce into an ovenproof dish and place the tilapia fillets in a single layer in the sauce. Bake in the oven for 12-15 minutes, until the fish is cooked. Drain the potatoes and mash with the remaining olive oil, milk, parsley and seasoning.
Divide the mash between four plates and top each one with a fillet of tilapia, drizzled with tomato sauce.