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 tablespoons 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. Stir in crushed tomatoes, bring to a simmer, and cook for 10 minutes until thickened.
03 - Heat remaining 1 tablespoon 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 cooked through at this stage.
04 - Transfer seared chicken and accumulated juices to the sauce. Simmer gently for 15 minutes, stirring occasionally. Add cream and sugar, continue simmering for 5 additional minutes. Adjust seasoning as needed and keep warm.
05 - Combine zucchini, carrot, potato, spring onions, peas, coriander, cumin, ground coriander, and chili flakes in a large bowl. Beat eggs separately and add to vegetables. Sprinkle flour, salt, and pepper over the mixture. Combine until just incorporated.
06 - 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 to 3 minutes per side until golden brown and crisp. Drain on paper towels.
07 - Spoon butter chicken into serving bowls or plates. Garnish generously with fresh chopped coriander. Arrange vegetable fritters alongside and serve immediately while hot.