Husk the tomatillos and rinse under warm water to remove the sticky residue. Preheat an outdoor grill to high heat. Grill the tomatillos, onions, and jalapeños until softened and lightly charred. Remove from the grill and allow to cool slightly.
Remove the stems from the jalapeños and add them to a blender along with the tomatillos and cilantro. Blend until smooth and set aside.
Lightly season the elk with granulated garlic powder and kosher salt. Heat the canola oil in a large skillet or Dutch oven over medium heat. Once hot, brown the elk on all sides, working in batches if needed. Remove the meat and set aside.
Dice the grilled onions and add them to the same skillet along with the minced garlic. Cook for 6–7 minutes, or until the onions are soft and fragrant. Stir in the cumin, coriander, oregano, and salt.
Carefully pour in the blended tomatillo mixture, followed by the beef stock. Return the browned elk to the skillet, cover, and reduce the heat to low. Simmer for 1 to 1½ hours, or until the elk is fork-tender.
Garnish with sour cream, crumbled cotija cheese, and avocado slices. Serve with tortilla chips or warm flour tortillas.