In a large sealable bag, combine cumin, cinnamon, 3/4 teaspoon salt, and 1/2 teaspoon pepper; add chicken and toss to coat.
In a large skillet, heat oil over medium-high. Cook chicken, skin side down, until golden, about 4 minutes; flip and cook 2 minutes.
In a 5-to-6-quart slow cooker, place onion, garlic, and ginger. Add chicken, skin side up, then top with tomatoes and their liquid. Cover and cook on high until chicken is tender, about 3 1/2 hours (or 6 hours on low).