Add remaining 2 Tbsp. oil to same pot and cook onion over medium-high, stirring occasionally, until softened and browned around the edges, 8–10 minutes. Sprinkle turmeric and cinnamon over and season with salt and pepper. Cook, stirring, until fragrant, about 30 seconds. Stir in tomato paste and cook, stirring, until slightly darkened in color, about 2 minutes. Add tomatoes; cook, still stirring, until tomatoes have burst and mixture is very thick, 7–9 minutes.