In a small skillet, toast the fenugreek leaves (or fenugreek seeds, if using) over medium heat, tossing them constantly, until quite fragrant, about 30 seconds.
Transfer the toasted leaves to a spice grinder or mortar and pestle and grind to fine powder.
Line a rimmed baking sheet with foil and spray with canola spray.
In a medium mixing bowl, stir together the yogurt, garam masala, salt, black salt, grated ginger, and ground fenugreek leaves.
Add the chicken thigh pieces to the bowl and, using clean hands, toss with marinade until evenly coated.
Transfer chicken to the prepared baking sheet, arranging pieces in a single, even layer. Set aside.