In a small bowl, use a fork to mix the olive oil, garlic, lemon zest, kosher salt, black pepper, smoked paprika, and ground cumin.
Pat the halibut fillets dry with a paper towel. Coat them all over with the olive oil mixture. You can use a pastry brush or your hands.
Preheat a grill pan, a gas grill, or an electric grill to medium-high and lightly grease it.
Grill the halibut over medium-high heat for 2 minutes per side, pressing gently on each fillet to ensure the fish has full contact with the grill.
Lower the heat to medium and keep cooking until the fish is cooked through and flakes easily with a fork, about 2 more minutes per side.
Lemon zest is the outside, yellow part of its peel. The inner peel, which is white in color, is bitter, so you should avoid it when you zest the lemon.
Grilled fish is delicious, but you don't want it charred. Two reliable ways to prevent charring are being vigilant about keeping the grill from getting too hot and turning the fish often while it cooks.
You can keep the leftovers in the fridge, in an airtight container, for up to 3 days. Reheat them gently, covered, in the microwave at 50% power.