Season chicken cubes with salt and pepper.
In a large skillet on medium-high heat, heat the olive oil. Add in the curry powder and stir until well blended. Add the onions and garlic and cook until the onions become soft and translucent, about 3-5 minutes.
Reduce heat to medium, add the chicken cubes and cook with the curry and onion mixture until the chicken is no longer pink and juices run clear, about 7 to 10 minutes.
Pour coconut milk and diced tomatoes into the pan, and stir to combine with the chicken. Cover and simmer, stirring occasionally until sauce thickens, approximately 30 to 40 minutes.
Serve over white rice and/or naan, and garnish with any herbs, if desired