In a 4- to 6-quart slow cooker, combine the tomatoes, onion, garlic, tomato paste, garam masala, ¾ teaspoon salt, and ¼ teaspoon pepper. Place the chicken on top of the vegetables, cover, and cook until the chicken is tender, on low for 7 to 8 hours or on high for 3 to 4 hours (this will shorten total recipe time).
In a small bowl, toss the cucumber and cilantro with the lemon juice and ¼ teaspoon each salt and pepper. Cover and refrigerate for up to 8 hours.
Twenty minutes before serving, cook the rice according to the package directions.
Just before serving, stir the cream into the chicken tikka masala. Serve over the rice with the cucumber relish.