In a small bowl, combine the garlic powder, ginger, thyme and red pepper flakes, if using. Then add salt and pepper to taste. Rub the spice mixture over the entire surface of the tenderloin.

In a large skillet, heat the oil over medium-high heat and brown the tenderloin on all sides. Transfer it to the slow cooker. Combine the broth, apple juice, soy sauce, and brown sugar in a small bowl and pour over the tenderloin. Cover and cook on low 6 to 8 or on high for 4 to 5 hours.

When the roast is tender and finished cooking, transfer to a serving platter and keep warm.

Meanwhile, pour the juices from the roast into a glass measuring cup; skim off the fat. You will need 2 cups of liquid. Pour these cooking juices into a saucepan. In a separate container, whisk or shake 3 tablespoons of cornstarch into ½ cup water until dissolved. Add this mixture to the cooking juices in the saucepan.

Heat over medium heat, stirring frequently, until the sauce bubbling and thickened. Cook and stir another couple of minutes. Season the sauce with salt and pepper to taste. Slice the tenderloin and serve the sauce alongside.