For the sauce: In a saucepan over medium-high heat, sweat the fennel, onions, and carrots in the olive oil. Cook until the vegetables are softened, 10 to 15 minutes. Next, deglaze with the white wine. Once the wine has reduced, add the stock and orange juice. Reduce the heat and allow the liquid to cook until it has reduced by half. Once reduced, add the tomatoes and garlic and allow the sauce to further simmer for 15 minutes. Taste and season. Remove saucepan from the heat, pour through a strainer, and return to the pan. Finish with the butter. Keep warm.
For the fish: Season the skin side of the fish fillets with the ground fennel, salt, and pepper, and then season the flesh side with salt and pepper.
In the oil in a nonstick saute pan over high heat until it reaches the smoking point. Reduce the heat and add the seasoned fish, skin side down, and cook for 2 to 3 minutes. Flip, reduce the heat, and continue to cook for 5 to 6 minutes, carefully spooning the hot oil over the fish to keep the skin crisp. Once the fillet is cooked, remove from the pan and serve with the sauce.