Set rack in top third of oven and preheat to 450°. Zest half of 1 lemon into a small bowl. Add zaatar, cumin, chile flakes, 1 tsp. salt, 1/4 tsp. pepper, and the oil; mix. Coat one side of fillets with paste.
Arrange halibut, paste side up, on a baking sheet and set on upper rack. Roast until fish is just cooked through, 6 to 10 minutes.
Put tahini, garlic, 2 tbsp. water, juice of 1/2 lemon, and remaining 1 tsp. salt and 1/4 tsp. pepper in a blender and whirl until smooth and pourable. Add more lemon juice to taste, and more water if you'd prefer a thinner sauce. Cut remaining lemon into wedges.
Serve fish with tahini sauce and lemon wedges.
*Find zaatar, a Middle Eastern spice blend of sesame seeds, sumac, thyme, and oregano, in your grocery store's spice aisle.
Wine pairing: A silky red like Crossbarn by Paul Hobbs 2012 Pinot Noir (Anderson Valley; $35); aromatic spices echo zaatar. --Sara Schneider