Preheat oven to 350°. Mix allspice, cardamom, cinnamon, nutmeg and 2 Tbsp. sumac in a small bowl to combine.

Season chicken all over with salt in a large bowl. Rub with half of spice mix, pressing and working it into the meat.

Heat ¼ cup oil in a large ovenproof skillet over medium-high. Cook chicken, turning occasionally, until golden brown on both sides and some fat is released, 3–5 minutes per side. Transfer chicken to a large plate, leaving fat in skillet.

Return skillet to medium-high heat. Cook onions, scraping any browned bits on bottom of skillet and adding a splash of water if pan gets dry, until softened and golden brown, 18–25 minutes. Sprinkle with remaining spice mix and continue to cook, stirring frequently, until fragrant, about 30 seconds. Add ¼ cup water, season with salt, and bring to a simmer. Nestle chicken back into skillet, skin side up, and transfer to oven. Bake, uncovered, until liquid is mostly evaporated and chicken is cooked through, 30–35 minutes. Let cool 10 minutes in skillet.

While chicken is cooking, toast pine nuts in remaining 1 Tbsp. oil in a small skillet over medium heat, stirring frequently, until golden brown and fragrant, about 3 minutes. Remove from heat and season with salt.

Arrange warmed bread on a platter and top with chicken and onions. Scatter pine nuts over and sprinkle with more sumac. Squeeze juice from one lemon half over chicken. Cut remaining lemon half into wedges to serve alongside.