In a mixing bowl, combine chicken, coconut milk, lemon juice, garam masala, cumin, coriander, turmeric, salt, and pepper. Mix well and let marinate for at least 20 minutes.

Heat coconut oil in a large skillet over medium heat. Add the marinated chicken and cook until browned and cooked through, about 10-15 minutes. Remove the chicken and set aside.

In the same skillet, add chopped onion and sauté until translucent, about 5 minutes. Add garlic and ginger, cooking for another minute until fragrant.

Stir in crushed tomatoes, coconut milk, cayenne pepper, and sugar. Bring to a simmer and cook for 10 minutes, allowing flavors to meld.

Return the cooked chicken to the skillet and stir to coat in the sauce. Cook for an additional 5 minutes. Garnish with fresh cilantro before serving.