In a large pot, cover the potatoes with water and bring to a boil. Salt the water and cook until tender, 10 to 12 minutes. Drain; return to the pot.
While the potatoes are working, season the halibut with the Old Bay seasoning, then wrap with the bacon. In a large nonstick skillet, heat 1/2 tablespoon oil over medium-high heat. Add the fish and cook, turning once, until the bacon is crisp, about 8 minutes.
In another large skillet, heat the remaining 2 tablespoons oil over high heat. Stir in the brussels sprouts, season with salt and pepper and cook for 3 minutes. Stir in the shallot, bell pepper, garlic and thyme. Add 1/2 cup chicken stock and cook until evaporated, 2 minutes. Turn off the heat and stir in the butter and tarragon.
Mash the hot potatoes with the remaining 1/4 cup chicken stock, the sour cream and chives; season with salt and pepper.
Serve the fish with the brussels sprouts, mashed potatoes and lemon wedges.