Pat dry the chicken with a paper towel. Make 3 to 4 small horizontal slits to each chicken piece. Apply salt and lemon juice to the chicken. Mix yogurt, red chili powder, garam masala, turmeric, ginger, and garlic. Apply evenly to the chicken and allow to marinate for 20 minutes or refrigerate if marinating ahead of time.
Add 1 cup of water to the Instant Pot insert. Place the trivet in the Instant Pot. Carefully place the marinated chicken over the trivet. Baste the chicken with oil.
Close the Instant Pot with the pressure valve to sealing. Pressure Cook for 8 minutes followed by 10-minute natural pressure release.
While the chicken is cooking, make the Raita - Whisk the yogurt in a medium bowl. Peel and grate the cucumber. Add grated cucumber to the yogurt. Sprinkle salt and cumin. Mix well. Garnish with cilantro, keep chilled in the refrigerator until ready to be served.
Open the Instant Pot. Take the chicken and the trivet out. Optional Step - Broil Chicken in a preheated oven or grill for 5 minutes. Cut chicken into 1-inch pieces and keep aside.
Carefully remove 10 to 12 whole lettuce leaves from the head of the lettuce. Take 1 lettuce leaf, place 4 to 5 pieces of chicken in the center of the leaf. Pour 1 to 2 tablespoon raita over the chicken and top with sliced onions.