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 develop flavors.
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 minute until fragrant.
03 - Stir crushed tomatoes into the onion mixture and bring to a simmer. Cook for 10 minutes, stirring occasionally, until sauce thickens and reduces slightly.
04 - Heat remaining 1 tbsp butter in a separate pan over medium-high heat. Add marinated chicken pieces and sear until lightly browned on all sides. Chicken does not need to be fully cooked through at this stage.
05 - Transfer seared chicken and any accumulated juices to the tomato sauce. Simmer gently for 15 minutes, stirring occasionally, until chicken is cooked through and flavors meld.
06 - Pour in heavy cream and add sugar. Simmer for an additional 5 minutes until sauce is rich and creamy. Taste and adjust seasoning as needed. Keep warm while preparing fritters.
07 - Combine grated zucchini, carrot, potato, sliced spring onions, thawed peas, chopped coriander, cumin, ground coriander, and chili flakes in a large mixing bowl.
08 - Beat eggs in a separate bowl and add to vegetable mixture. Sprinkle flour, salt, and pepper over the top. Mix until just combined, being careful not to overwork the batter.
09 - Heat vegetable oil to approximately 1/2 inch depth in a large skillet over medium heat. Drop heaped tablespoons of batter into hot oil, flattening gently with the back of the spoon. Cook 2-3 minutes per side until golden brown and crisp. Drain on paper towels.
10 - Ladle butter chicken into serving bowls or plates. Garnish generously with fresh chopped coriander. Accompany with hot vegetable fritters on the side.