Season the pork generously with salt and pepper. Add the garlic, chile powder and cumin, and mix well, massaging the seasoning into the meat with fingers. Let the meat marinate for at least an hour, or refrigerate overnight.
Heat the oil in a wide cast iron skillet over medium heat. When the oil looks wavy, add the meat and let it sizzle, stirring occasionally, until lightly browned and cooked through, 5 to 7 minutes. Turn off the heat and keep meat warm
Heat the tortillas on an ungreased hot griddle or cast iron pan. Turn them once or twice until hot and slightly puffed. As they come off the griddle, stack them on a cloth napkin to steam, with another napkin on top.
To assemble the tacos, stack 2 hot tortillas on a plate. Spoon on a little sizzled pork, then top with a spoonful of tomatillo salsa. Garnish with onion and cilantro sprigs. Serve and eat immediately.