For the potatoes and tomato concasse, cut a small cross into the bottoms of the tomatoes with a small, sharp knife and blanch them in a pan of boiling water for about ten seconds, then drain and plunge into ice-cold water. Peel the tomatoes and discard the skins.
Cut the peeled tomatoes into quarters and remove the seeds. Pat the tomato pieces dry with a clean tea towel and cut into small dice.
Cut the new potatoes into barrel shapes using a small, sharp knife. Cook the potatoes in a pan of boiling salted water for 15 minutes, or until tender. Season, to taste, with salt and freshly ground black pepper and add a knob of the butter. Stir in the finely chopped tomatoes and the chives and set aside.
For the stone bass fillets, preheat the oven to 200C/400F/Gas 6.
Lay two layers of foil long enough to fold over to enclose the fish fillets onto a work surface, shiny-side up. Lay a slightly smaller piece of parchment paper on top of the foil, place a fish fillet on top then fold over the edges of the foil to secure the parchment. Repeat until you have four parcels.
Season the fish with salt and freshly ground black pepper. Add a splash of wine (about one tablespoon), a squeeze of lemon and top with a small knob of butter. Place the baby leeks and a few asparagus spears on top of the fish. Scatter over the shallot rings and the tarragon.
Fold up each package and seal it on top, leaving some gaps for the steam to escape. Place the foil packages on a flame-proof baking tray, then place the tray on the hob over a medium heat for 1-2 minutes. Carefully remove the tray from the hob and place into the oven for 6-8 minutes, or until the fish is cooked through.
To serve, open the packets and carefully transfer the fish and vegetables to serving plates. Drizzle over any of the juices left behind in the packet, and serve with the new potatoes and tomato concasse alongside.