Marinate 250 grams of boneless chicken pieces with 100 grams of yogurt, 1 tablespoon of ginger-garlic paste, 1 teaspoon of red chili powder, 1 teaspoon of garam masala, 1/2 teaspoon of turmeric powder, 1/2 teaspoon of cumin powder, salt to taste, and 1 tablespoon of lemon juice. Mix well and refrigerate for at least 1 hour.
Preheat a grill or oven to 200°C. Skewer the marinated chicken pieces and grill for 15-20 minutes, turning occasionally, until cooked through and slightly charred. Remove from the grill and let it cool.
In a bowl, mix 1/4 cup of mayonnaise, 1/4 cup of hung curd, 1/2 teaspoon of black pepper powder, and salt to taste. Stir well to make the sandwich spread.
Take 4 slices of bread and spread the prepared mayonnaise mixture on one side of each slice.
Place the grilled chicken tikka pieces on two of the bread slices, and top with sliced onions, tomatoes, and cucumber.
Cover with the remaining bread slices, mayonnaise side down, and press gently.
Heat a pan or sandwich maker, and toast the sandwiches until golden brown and crispy on both sides.