Heat oil in a large Dutch oven over high; add chicken, cauliflower, onion, salt, and pepper; cook, stirring occasionally, until chicken is starting to brown and vegetables are softening, about 5 minutes. Stir in ginger and garam masala; cook, stirring constantly, until fragrant, about 1 minute.
Reduce heat to medium, and stir in tomato sauce and coconut milk; bring to a boil, stirring occasionally. Boil, stirring occasionally, until reduced and slightly thickened, about 10 minutes. Serve over rice; garnish with cilantro, if using.
