In a bowl, mix 1 cup yogurt, 2 tablespoons lemon juice, 2 tablespoons tandoori masala, 1 teaspoon turmeric, 1 teaspoon chili powder, and salt. Add 500g chicken pieces and coat well. Refrigerate for 1-2 hours.
Preheat your oven to 200°C (400°F). Line a baking sheet with parchment paper for easy cleanup.
Place the marinated chicken on the baking sheet. Bake for 20-25 minutes, turning halfway, until cooked through and slightly charred.
In a skillet, heat 2 tablespoons butter and 1 tablespoon oil over medium heat. Add 1 finely chopped onion and sauté until golden brown.
Stir in 2 pureed tomatoes and cook for 5-7 minutes until thickened, stirring occasionally.
Add the baked chicken to the sauce along with any juices from the baking sheet. Mix well to coat the chicken.
Pour in 1/2 cup heavy cream and sprinkle 1 teaspoon garam masala. Stir well, simmer for 5 minutes, and garnish with fresh coriander. Serve hot!