Charred Sweet Potatoes with Toum

Chill oil in freezer 30 minutes (this helps the sauce emulsify). Pulse garlic in a blender with lemon juice, 1/4 cup cold oil, and 1 Tbsp. ice water until smooth. With motor running, very gradually and steadily stream in 1/2 cup oil (you don't want to risk having the emulsion split). Scrape down sides and continue to blend, slowly adding remaining 1/4 cup oil, until a slightly thick sauce forms (it should cling to a spoon). Add 1 more Tbsp. ice water, season with kosher salt, and pulse again to combine. This whole process will occupy 8–10 minutes. Take your time! Transfer toum to a medium bowl.
Place a rack in bottom third of oven and preheat to 400°F. Place sweet potatoes in a large heavy skillet, preferably cast iron. The pan should be large enough so that sweet potatoes cover only half of the pan. Pour in just enough water to barely coat bottom of pan. Cover tightly with foil and bake sweet potatoes until tender, 30–35 minutes. Transfer to a cutting board and let cool; reserve skillet. Slice sweet potatoes in half lengthwise.
Return reserved skillet to rack and heat 20 minutes. Remove skillet from oven and swirl 2 Tbsp. butter around in pan to coat. Add sweet potatoes, cut side down, and roast until edges are browned underneath and crisp, 18–25 minutes.
Spoon some toum on a plate or in shallow bowl and arrange sweet potatoes on top (save extra toum for another use). Stir remaining 2 Tbsp. butter and honey in skillet and place over medium heat to warm. Drizzle honey butter over sweet potatoes. Sprinkle with nigella seeds, if using, and sea salt.