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, approximately 6 minutes. Add garlic and ginger, cook for 1 minute until fragrant.
03 - Stir in crushed tomatoes and bring to a simmer. Cook for 10 minutes, stirring occasionally, until sauce thickens noticeably.
04 - Heat 1 tbsp butter in another pan over medium-high heat. Add marinated chicken and sear until lightly browned on all sides, about 3-4 minutes. The chicken does not need to be 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 fully cooked.
06 - Add cream and sugar to the sauce. Simmer for 5 additional minutes to allow flavors to meld. Adjust seasoning to taste. Keep warm until serving.
07 - In a large bowl, combine grated zucchini, carrot, potato, spring onions, peas, coriander, cumin, ground coriander, and chili flakes.
08 - Beat eggs in a separate bowl, then add to vegetable mixture. Sprinkle in flour, salt, and pepper. Mix until just combined, being careful not to overwork the batter.
09 - Heat 1/2 inch vegetable oil in a large skillet over medium heat. Drop heaped tablespoons of batter into the pan, flattening gently with the back of the spoon. Cook for 2-3 minutes per side until golden brown and crisp. Drain on paper towels.
10 - Spoon butter chicken into serving bowls or plates. Garnish generously with fresh coriander. Arrange vegetable fritters alongside and serve immediately while hot.