Heat oven to 350 degrees. Heat a large, oven-safe skillet over medium. Add butter and oil, and cook until butter melts and starts to sizzle, 1 to 2 minutes. Stir in olives, garlic and fennel or coriander, and cook until fragrant, 1 to 2 minutes.
Step 2
Season fish fillets with salt and pepper, then nestle them in one layer in the skillet. Carefully spoon some of the butter mixture over the fish, basting the fillets, then transfer pan to the oven.
Step 3
Bake until fish is opaque and flaky, 8 to 13 minutes, basting halfway through. Remove from oven and transfer fillets to serving plates.
Step 4
Squeeze a wedge or two of lime into the butter mixture and spoon sauce over fish. Top with black pepper, cilantro and dill, with extra lime wedges on the side for squeezing.