In a 4- to 6- quart slow cooker, combine the tomatoes, onion, garlic, tomato paste, garam masala, salt, and pepper. Place the chicken on top of the vegetables, cover and cook until the chicken is tender, on low for 7-8 hours or on high for 3-4 hours.
In a small bowl, toss the cucumbers and cilantro with lemon juice and 1/3 tsp salt and peper. Cover and refrigerate for up to 8 hours.
20-40 minutes before serving, cook the rice according to the package directions.
Just before serving, stir the cream into the chicken tikka masala. Server over rice with the cucumber relish