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. Add chopped onion and sauté until golden brown, approximately 6 minutes. Add minced garlic and grated ginger, cooking for 1 additional minute until fragrant.
03 - Pour crushed tomatoes into the pan and bring to a gentle simmer. Cook for 10 minutes, stirring occasionally, until sauce thickens noticeably.
04 - While sauce simmers, heat 1 tbsp butter in another pan over medium-high heat. Add marinated chicken pieces and sear until lightly browned on all sides. Chicken does not need to cook through at this stage.
05 - Transfer seared chicken and accumulated juices to the tomato sauce. Simmer gently for 15 minutes, stirring occasionally to prevent sticking.
06 - Stir in heavy cream and sugar. Continue simmering for 5 minutes until sauce reaches desired consistency. Adjust seasoning with salt if needed. Keep warm over low heat.
07 - In a large bowl, combine grated zucchini, carrot, potato, sliced spring onions, thawed peas, chopped coriander, cumin, ground coriander, and chili flakes.
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 1/2 inch vegetable oil in a large skillet over medium heat. Drop heaped tablespoons of batter into pan, flattening gently with back of spoon. Fry for 2-3 minutes per side until golden brown and crispy. Drain on paper towels.
10 - Spoon butter chicken into serving bowls. Garnish with fresh chopped coriander. Arrange vegetable fritters alongside. Serve immediately while hot.