Preheat oven to 450-degrees. In a large cast iron skillet, add potatoes, olive oil, half of the dill and season with salt and pepper. Roast for 15-20 minutes, or until potatoes are almost tender.
Blot halibut fillets with a paper towel to remove any moisture and cover with the lemon juice. Let halibut soak in lemon juice while potatoes are cooking.
Remove pan from the oven and top with halibut fillets, olives and stir in garlic. Drizzle halibut with a little olive oil and top with remaining dill, lemon zest, and lemon juice. Bake for 10 minutes, or until fish flakes with a fork. Serve immediately. Garnish with extra dill, if desired.