In the base of your slow cooker, combine the crushed tomatoes and brown sugar. Stir.

In a large nonstick skillet, heat the oil over medium-high heat. Add the onion and cook until softened, 5 to 7 minutes. Add the garlic, ginger, tomato paste, red pepper flakes, garam masala, salt, and pepper and cook until fragrant, about 1 minute. The mixture will be a thick paste.

Transfer the onion and spice mixture to the slow cooker and stir well to combine with the crushed tomatoes. Add the chicken breasts, pushing them down enough to cover them with the sauce. Cover and cook on low for 4 hours.

Remove the chicken breasts from the slow cooker and cut into smaller chunks. Stir the half and half into the tomato sauce in the slow cooker. Return the cut-up chicken to the slow cooker and stir. Serve with the chopped fresh cilantro.