Heat the oven to 220 C / 425 F with a rack in the center. Line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment.
Peel off any wilted outer leaves. Stand the cabbage upright on its flat stem end and slice straight down through the core into slabs 2.5 cm / 1 inch thick, starting at the middle. Keep the 4 widest slabs and save the rounded outer pieces for another use.
Whisk the olive oil, turmeric, smoked paprika, garlic powder, 3/4 tsp salt, and pepper in a small bowl. Brush the mixture over both sides of each steak, working it into the cut edges. Arrange the steaks on the pan with about 3 cm / 1 inch between them.
Roast for 20 minutes, until the undersides are deep golden brown. While they roast, stir together the yogurt, 1 tbsp lemon juice, grated garlic, and a pinch of salt in a small bowl.
Slide a wide spatula fully under each steak and flip it. Roast for 15 minutes more, until the edges are dark brown and crisp and the tip of a paring knife slides into the thickest part with little resistance.
Squeeze a lemon wedge over the hot steaks and scatter with parsley. Serve right away with the garlic yogurt spooned over the top and extra wedges on the side.