Place the cubed chicken, diced onion, minced garlic, and grated ginger into the slow cooker basin.
In a medium bowl, whisk together the tomato sauce, tomato paste, garam masala, turmeric, cumin, coriander, paprika, salt, and pepper.
Pour the spice and tomato mixture over the chicken and aromatics, stirring to ensure even coating.
Cover and cook on high for 3 to 4 hours or on low for 7 to 8 hours until the chicken is tender and cooked through.
Stir in the full-fat coconut milk and lemon juice 15 minutes before serving to incorporate and heat through.
Garnish with fresh cilantro and serve over basmati rice or with naan bread.