Preheat the oven to 200°C and place an ovenproof dish inside.
Stir together garlic, onion, potatoes, thyme, and 50ml olive oil in a bowl with seasoning.
Transfer the mixture to the preheated dish and bake for 12-15 minutes.
Stir in 100ml wine and tomatoes and bake for another 5 minutes until potatoes are tender.
Heat 1 tbsp oil in a pan and fry seasoned mullet fillets for 2-3 minutes per side.
Boil 100ml wine, lemon juice, and rosemary in a pan until reduced by half.
Add cream and simmer until reduced by half, then season to taste.
Serve fish fillets over the gratin and drizzle with the rosemary sauce.