Heat a skillet over medium heat and toast the garam masala and cumin seeds for 1-2 minutes until fragrant, stirring constantly.
Add a splash of oil to the skillet, then sauté the minced garlic and grated ginger for about 1 minute until aromatic and slightly golden.
Add the chopped onion to the skillet and cook for 5-7 minutes until translucent and beginning to caramelize, filling the air with sweet, savory aroma.
Transfer the sautéed aromatics to the slow cooker, along with the tomato puree, Greek yogurt, toasted spices, chili powder, salt, and pepper. Stir well to combine everything evenly.
Place the chicken chunks into the mixture, pressing them down so they are submerged in the sauce.
Set the slow cooker to low and cook for 6 hours, avoiding opening the lid during the first few hours to keep moisture inside.
Once cooking is complete, check that the chicken is tender and easily shreddable. If the sauce is too thin, uncover and simmer on high for 30 minutes to thicken naturally.
Stir in the Greek yogurt off the heat for a creamy, tangy finish, then add a squeeze of lemon juice for brightness. Let sit for 10 minutes to meld flavors.
Garnish with fresh cilantro if desired, then serve hot over rice or with naan, enjoying the tender chicken coated in a rich, fragrant sauce.
