Place the sliced onions in the bottom of a 6- or 8-quart slow cooker.
In a large bowl, combine the chicken, red bell pepper, carrots, tomato sauce, tomato paste, lemon juice, garlic, ginger, curry powder, garam masala, and salt. Toss until well blended.
Add the mixture to the slow cooker, spreading evenly over the onions. Dot the tops with butter pieces.
Cover and cook on high for 4 ½ hours or on low for 6 ½ hours.
Once cooked, turn off the slow cooker and stir everything together.
Allow the mixture to cool for 5-10 minutes. Stir in the Greek yogurt and half-and-half, mixing until smooth. (Do not skip the cooling step to prevent curdling.)
Serve over cooked rice or cauliflower rice, with naan if desired. Garnish with cilantro, sesame seeds, and a lime wedge.