Marinate chicken with yogurt, 1 tsp ginger, 1 tsp garlic, and 0.5 tsp salt. Chill 15 minutes.
Soak cashews in the warm water for 10 minutes, then blend until smooth; set aside.
Warm ghee in a large skillet over medium. Add onions and cook, stirring, until deep golden, 12–15 minutes.
Stir in remaining ginger and garlic. Cook until fragrant, about 30 seconds.
Sprinkle in coriander, cumin, turmeric, garam masala, cardamom, cayenne, remaining salt, and black pepper. Bloom 30 seconds.
Add marinated chicken and any juices. Cook, stirring, until no longer pink on the outside, 3–4 minutes.
Pour in broth. Bring to a gentle simmer, cover, and cook until chicken is tender, 12–15 minutes.
Reduce heat to low. Stir in cashew paste and heavy cream. Simmer uncovered until silky and slightly thick, 4–6 minutes.
Season with sugar if using, then finish with lemon juice. Rest 2 minutes and garnish with cilantro.