Place the diced onion, minced garlic, and minced ginger in the bottom of the slow cooker. Lay the chicken breasts on top in a single layer. Pour the tomato sauce over the chicken. Add the butter pieces, then sprinkle in the garam masala, cumin, paprika, turmeric, and salt. Cover and cook on low for 6 hours. Remove the chicken breasts and shred or slice them. Stir the heavy cream into the sauce until smooth. Return the chicken to the sauce and stir to coat. Serve the butter chicken over or alongside the warmed naan, topped with chopped cilantro.