In small bowl, combine and mix well the cardamom, coriander, cumin, fennel, 1 teaspoon salt and 1/2 teaspoon black pepper. Reserve 1 teaspoon spice mixture. Spread remaining spice mixture on dinner plate. Brush chicken thighs evenly with 1 tablespoon olive oil. Press chicken thighs into spice mixture, coating evenly. Set chicken aside.
Place grated cucumber on several layers of paper towels; top with more paper towels. Pat well to remove excess water from cucumber. In medium bowl, stir together cucumber, yogurt, lemon zest, garlic, cayenne and reserved one teaspoon spice mixture. Season with remaining 1/2 teaspoon salt and 1/2 teaspoon black pepper. Set aside.
Warm the remaining 1 tablespoon olive oil in a large skillet over medium-high heat. Add chicken and cook in single layer until skin is golden brown, about 3-5 minutes. Reduce the heat to low, turn chicken, and cook until done, about 4-55 minutes longer.
Place chicken thighs on serving platter; top with yogurt sauce and serve immediately.