In a medium bowl, mix Greek yogurt, lemon juice, garam masala, cumin, turmeric, salt, and black pepper. Add chicken and coat. Let rest 15–20 minutes.
Heat 2 tablespoon butter in a large skillet over medium-high heat. Add chicken and cook 5–6 minutes until golden and cooked through. Remove and set aside.
In the same skillet, melt remaining butter and cook onion until soft and golden. Add garlic and ginger and cook 1 minute.
Stir in garam masala, cumin, coriander, turmeric, cayenne, and fenugreek. Cook 30–60 seconds until aromatic.
Add crushed tomatoes, tomato paste, and sugar. Simmer 5–6 minutes until slightly thickened.
Lower heat and stir in heavy cream. Return chicken to the pan and simmer 3–4 minutes. Season with salt to taste.
Serve hot over basmati rice with naan and garnish with fresh cilantro.