Preheat the oven to 400 degrees F.

Put the leeks in a roasting pan. Toss the tomatoes with 2 tablespoons grapeseed oil, vinegar, rosemary, thyme, and salt in a small bowl and add it to the leeks and toss. Put the pan in the oven, uncovered, and roast for 20 minutes, stirring after 10 minutes. Remove from oven when the leeks have softened to touch or when the tomatoes have begun to shrivel.

In a large nonstick pan, over medium-high heat, add 2 tablespoons butter and allow the butter to melt. Drizzle the halibut with remaining oil, and season with salt and pepper, to taste. Put the halibut in the buttered pan, skin side down, and reduce the heat to medium. Allow the fish to cook for 5 minutes, carefully flip and repeat the cooking process on the second side. After the second side has cooked, remove the halibut to a plate and keep warm. Return the pan to high heat. Stir in the wine with a wooden spoon, to deglaze the pan. Add the lemon juice and chicken stock. This pan sauce should reduce by half, or until thickened. Once the sauce has reduced, remove the pan from the heat and stir in the remaining butter. Arrange the halibut on serving plates, drizzle with the sauce and serve with the roasted leeks and tomatoes alongside.