Toss 3 sweet potatoes (cut into 1-inch-thick rounds) with 2 tablespoons olive oil and 1 teaspoon ras el hanout on a baking sheet. Season with salt, pepper and a pinch of cayenne. Roast at 400˚, 30 minutes. Smash the potatoes, then roast at 475˚ until crisp, 25 to 30 minutes. Drizzle with 1/4 cup plain yogurt mixed with 1/4 teaspoon each lemon juice and zest and a pinch each of salt and pepper.