Preheat the oven to 150ºC . Carefully cut the out core of each tomato, removing a small, narrow plug and leaving the tomato intact. Blanch the tomatoes in boiling water for a few seconds; drain under cold water and slip off their skins. Cut a lid from the smooth bottom of each tomato (opposite the core) and set aside. With a melon scoop or spoon, carefully hollow out the pulp and seeds from the tomatoes, leaving a neat shell. Place the tomatoes and their lids in a small buttered baking dish, sprinkle with salt and pepper, set aside. In a small skillet, heat 2 tablespoons of butter and saute the sliced onions over medium heat until softened but not browned, 5 to 7 minutes. Spoon cooked onions into the hollowed tomatoes, dividing evenly. On a work surface, gently flatten the sole fillets, tapping them with the side of a wide knife blade. Carefully roll up each fillet, beginning with the small (tail) end and with the smooth, shiny skinned surface on the inside. Lightly butter a heat-proof shallow baking dish in which the fillets will fit compactly. Scatter shallot into the dish, then arrange rolled fish fillets over shallot. Salt and pepper the fish lightly and pour wine over all. Cover the dish with buttered parchment or waxed paper, buttered side down. Place the pan over medium heat and bring almost to a boil. Place the pan of fish and the pan of tomatoes in the oven. Bake until fillets are just cooked through, 6 to 8 minutes. DO NOT OVERCOOK. Remove the pans from the oven. With a slotted spatula, lift each fillet, draining all of the liquid back into the pan, and place each fillet upright in a tomato. Place the pan of fish-cooking liquid over high heat and boil the liquid until it's reduced to a few tablespoons of syrupy liquid. Reduce the heat to very low. Whisk in the remaining dozen tablespoons of butter, 1 or 2 pieces at a time, adding more only when the previous addition has become creamy and smooth. Strain this sauce into a small bowl, stir in the parsley and chives and season to taste with salt and pepper. Place each stuffed tomato on a warm serving plate. Spoon some of the sauce over each fillet and around each tomato. Gently replace the lids and serve immediately.