In a large saucepan of 2 cups water, cook rice according to package instructions; set aside.
Melt butter in a large skillet over medium heat.
Season chicken with salt and pepper, to taste. Add chicken and onion to the skillet and cook, stirring occasionally, until golden, about 4-5 minutes; set aside.
Stir in tomato paste, ginger, garlic, garam masala, chili powder and turmeric until well combined, about 1-2 minutes.
Stir in diced tomatoes and chicken stock. Bring to a boil; reduce heat and simmer, stirring occasionally, until slightly thickened, about 10 minutes.
Stir in heavy cream, lemon juice and chicken until heated through, about 1 minute.
Place rice and chicken mixture into meal prep containers, garnished with cilantro, if desired.