Step 1: Prepare the Chicken. Cut the chicken thighs into 1-inch bite-sized pieces. Pat them dry thoroughly with paper towels.
Step 2: Make the Marinade. In a large bowl, combine Greek yogurt, ginger-garlic paste, lemon juice, garam masala, ground cumin, turmeric powder, chili powder, salt, and black pepper. Mix thoroughly until well combined.
Step 3: Marinate the Chicken. Add the chicken pieces to the marinade, tossing until every piece is evenly coated. Cover the bowl and refrigerate for at least 30 minutes, or ideally 2-4 hours, or even overnight for maximum flavor and tenderness.
Step 4: Cook the Chicken Tikka. Heat olive oil in a large non-stick skillet or cast-iron pan over medium-high heat. Add the marinated chicken in a single layer, making sure not to overcrowd the pan (cook in batches if necessary). Cook for 3-5 minutes per side, until beautifully charred in spots and cooked through (internal temperature of 165°F / 74°C). Remove and set aside.
Step 5: Prepare Your Toppings. While the chicken cooks, thinly slice the red onion and chop the fresh cilantro. If making the creamy yogurt sauce, whisk together yogurt/sour cream, lime juice, a pinch of sugar, and a pinch of salt.
Step 6: Warm the Tortillas. Heat your tortillas using your preferred method: a dry skillet over medium heat, briefly over an open gas flame, or wrapped in a damp paper towel and microwaved until soft and pliable.
Step 7: Assemble Your Chicken Tikka Tacos. Fill each warm tortilla generously with the cooked chicken tikka. Top with sliced red onion, fresh cilantro, and a drizzle of the creamy yogurt sauce (if using). Serve immediately with extra lime wedges on the side.