In a mixing bowl, combine the plain yogurt, tandoori spice mix, lemon juice, minced garlic, grated ginger, and salt. Whisk everything together until smooth and well combined.
Add the chicken thighs to the bowl, turning to coat each piece thoroughly in the marinade. Cover the bowl with plastic wrap and refrigerate for at least 1 hour, preferably 4 hours or overnight.
Preheat your oven to 400°F (200°C). Line a baking sheet or roasting pan with foil or parchment paper for easy cleanup.
Arrange the marinated chicken thighs on the prepared baking sheet in a single layer. Roast for 25-30 minutes, or until the internal temperature reaches 165°F (74°C) and the edges are slightly charred. Optionally broil for 2-3 minutes for extra char.
While the chicken roasts, rinse the basmati rice under cold water until the water runs clear. Soak the rice in water for about 20 minutes.
Drain the soaked rice and add it to a medium saucepan with 4 cups of water and 2 tablespoons of vegetable oil. Bring to a boil over medium-high heat, then reduce to low, cover, and simmer for 15 minutes. Remove from heat and let it rest, covered, for 10 minutes.
Fluff the basmati rice with a fork and transfer to serving plates. Place the roasted tandoori chicken thighs on top or alongside. Garnish generously with fresh cilantro.