Preheat the oven to 190°C/375°F/gas 5. Finely slice the whole fennel, peel and finely slice the garlic and lightly crush the fennel seeds. Place a medium pan over a medium heat and add the olive oil and half the butter. Once hot, add the fennel and fennel seeds and fry gently for around 5 minutes. Add the garlic and continue cooking for around 5 minutes, or until the fennel is soft, golden brown and sweet. Set aside to cool.
Meanwhile, place a medium pan over a high heat and cook the potatoes for around 5 minutes, or until parboiled. Drain and set aside. Finely chop all the herbs together and add to a bowl along with remaining butter, then mix well until combined and set aside. Ensure the sea bass is clean inside and out, pat dry with kitchen paper and make 3 diagonal cuts into one side of the fish down to the bone. Stuff the cuts with the herb butter and fill the cavity of each fish with 2 slices of lemon and the cooked fennel.
Gently transfer the fish to the inside of the foil bag or onto the foil. Add in the whole potatoes, capers, tomato sauce and white wine. Seal the foil bag at the end or, if you're using foil, fold it up like a parcel.
Place the foil bags onto a baking tray and pop into the oven. Bake for 20-25 minutes, or until cooked through. Remove from the oven and allow to sit for 2-3 minutes, then place onto a serving plate, carefully open and enjoy with a crisp green salad or steamed vegetables.