01 - Whisk yogurt, lemon juice, garam masala, cumin, coriander, chili powder, turmeric, and salt in a bowl. Add chicken pieces, toss thoroughly to coat. Cover and refrigerate for 30 minutes.
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.
03 - Stir in crushed tomatoes and bring to a simmer. Cook for 10 minutes, stirring occasionally until sauce thickens.
04 - Heat 1 tbsp butter in another pan over medium-high heat. Add marinated chicken pieces and sear until lightly browned on all sides, about 3-4 minutes. Transfer chicken and juices to the tomato sauce.
05 - Simmer chicken in sauce gently for 15 minutes. Add heavy cream and sugar, continue simmering for 5 minutes. Adjust seasoning to taste. Keep warm.
06 - In a large bowl, combine grated zucchini, carrot, potato, spring onions, peas, coriander, cumin, ground coriander, and chili flakes.
07 - Beat eggs in separate bowl and add to vegetables. Sprinkle flour, salt, and pepper over mixture. Fold until just combined, do not overmix.
08 - Heat 1/2 inch vegetable oil in large skillet over medium heat. Drop heaped tablespoons of batter into pan, flatten gently. Cook 2-3 minutes per side until golden brown and crisp. Drain on paper towels.
09 - Ladle butter chicken into serving bowls. Garnish with fresh coriander. Arrange vegetable fritters alongside. Serve immediately while hot.