In a large bowl, mix buttermilk, chopped onion, pressed garlic, salt, sugar, cumin, and black pepper. Stir until the salt and sugar have completely dissolved.
Cut the chicken into bite-sized pieces. Add to the marinade bowl and mix thoroughly with clean hands or a spoon until all pieces are well coated.
Cover the bowl with plastic wrap or a lid. Refrigerate for 8 to 24 hours.
Drain the marinated chicken through a sieve to remove excess buttermilk. Optionally retain the onion and garlic pieces to cook alongside the chicken.
Heat a non-stick pan over medium-high heat (add a small amount of oil if using a regular pan). Fry the drained chicken for about 5 minutes, stirring occasionally, until the internal temperature reaches 74°C (165°F).