Heat the oil in a large sauté pan that has a lid over medium-high heat. Add the onion, chili pepper, and salt and sauté until the onion starts to soften, about 2 minutes.

Add the chicken in an even layer and cook, stirring occasionally, until slightly browned in spots.

Stir in the garlic, ginger, curry powder, chili powder, cumin, turmeric, black pepper, and garam masala (if using). Cook, stirring constantly, until the chicken is evenly coated in the ground spices, about 1 minute.

Pour in the chopped tomatoes and stir to combine.

Add the sweet potatoes and coconut milk, scraping up any browned bits. Bring the curry to a gentle boil, then reduce to a simmer. Cover and simmer for about 15 minutes, stirring occasionally.

Taste and season with a pinch more salt and pepper, if needed.

Add the spinach, cover the pan again, and cook until the spinach is wilted and sweet potatoes are fork-tender, about 5 minutes more.

Turn off the heat, drizzle in lemon juice, and stir in chopped cilantro. Serve hot over rice or with flatbread garnished with more cilantro, if desired.