Heat oil in a sauté pan over medium heat. Add the chopped onion and cook until it becomes soft, fragrant, and slightly caramelized, about 5 minutes.
Add the minced garlic and curry powder to the pan, stirring constantly for about 1 minute until fragrant and slightly toasted.
Transfer the sautéed onion, garlic, and spices to the slow cooker. Place the chicken pieces on top.
Pour the diced tomatoes and coconut milk over the chicken, ensuring everything is submerged. Season with salt and pepper to taste.
Cover the slow cooker with the lid and set it to low. Let it simmer for about 6 hours, or until the chicken is tender and easily shredded, filling your kitchen with a warm, spicy aroma.
Once cooked, remove the chicken and shred it with two forks. Return the shredded chicken to the sauce and stir well to coat evenly.
Taste the curry and adjust seasoning with more salt, pepper, or a squeeze of lemon/lime if desired. Garnish with chopped fresh cilantro or basil for a bright finishing touch.
Serve the chicken curry hot over rice or with warm naan bread, enjoying the rich, silky sauce and tender chicken with every bite.