01 - Combine yogurt, lemon juice, garam masala, cumin, coriander, chili powder, turmeric, and salt in a bowl. Add chicken pieces and toss to coat thoroughly. Cover and refrigerate for at least 30 minutes to allow flavors to penetrate.
02 - Melt 2 tbsp butter in a large pan over medium heat. Sauté onion until golden brown, approximately 6 minutes. Add minced garlic and grated ginger, cooking for 1 additional minute until fragrant.
03 - Stir in crushed tomatoes and bring to a gentle simmer. Cook for 10 minutes, stirring occasionally, until the sauce thickens considerably.
04 - While sauce simmers, heat remaining 1 tbsp butter in a separate pan over medium-high heat. Remove chicken from marinade and sear until lightly browned on all sides. Chicken does not need to be cooked through at this stage.
05 - Transfer seared chicken and any accumulated juices to the tomato sauce. Reduce heat to low and simmer gently for 15 minutes, stirring occasionally to prevent sticking.
06 - Pour in heavy cream and add sugar. Simmer for 5 additional minutes until the sauce achieves a rich, velvety consistency. Taste and adjust seasoning as needed. Keep warm.
07 - Combine grated zucchini, carrot, potato, spring onions, peas, coriander, cumin, ground coriander, and chili flakes in a large bowl. In a separate bowl, beat eggs, then add to vegetables. Sprinkle flour, salt, and pepper over mixture. Stir until just combined.
08 - Heat 1/2 inch vegetable oil in a large skillet over medium heat. Drop heaped tablespoons of batter into hot oil, flattening gently with the back of the spoon. Fry for 2-3 minutes per side until golden brown and crispy. Drain on paper towels.
09 - Spoon butter chicken into serving bowls or plates. Garnish generously with fresh chopped coriander. Arrange hot vegetable fritters alongside the chicken.