Stir the cornstarch, stock and black pepper in a small bowl until the mixture is smooth.
Spray a 10-inch skillet with the cooking spray and heat over medium-high heat for 1 minute. Add the pork and cook until it's well browned both sides. Remove the pork from the skillet. Remove the skillet from the heat.
Step 3 - Add Onion
Spray the skillet with the cooking spray and heat over medium heat for 1 minute. Add the onion and cook until it's tender-crisp.
Step 4 - Final
Stir in the cornstarch mixture. Cook and stir until the mixture boils and thickens. Return the pork to the skillet. Reduce the heat to low. and cook for 5 minutes or until the pork is cooked through.