Trim any silver skin from the tenderloins using a sharp knife. Silver skin is the thin, shiny membrane that runs along the side of the meat. It does not break down during cooking and will make the meat tough if left on. Pat both tenderloins dry with paper towels.

Mix the salt, pepper, garlic powder, and onion powder in a small bowl. Rub this seasoning all over both tenderloins on all sides. If you have time, heat a skillet over medium-high heat with olive oil and sear the tenderloins for 2 to 3 minutes per side until lightly browned. This step is optional but adds a deeper flavor to the finished dish.

In a small bowl or measuring cup, whisk together the apple cider, Dijon mustard, brown sugar, and minced garlic. Pour this mixture into the bottom of the slow cooker insert.

Place both tenderloins in the slow cooker, nestling them into the cider mixture. They can overlap slightly at the ends if needed. Cover and cook on LOW for 4 to 5 hours. Do not cook on HIGH, as tenderloin can dry out quickly on a higher setting.

Start checking the internal temperature at the 4-hour mark using an instant-read thermometer inserted into the thickest part of one tenderloin. You are looking for 145 degrees F. Pull the meat at 145, not higher. Resting will carry the temperature up a few degrees on its own.

Transfer the tenderloins to a cutting board and let them rest for 5 minutes before slicing. Slice against the grain into medallions about 1/2 inch thick. Spoon the pan juices from the slow cooker over each serving. If you want a slightly thicker glaze, pour the juices into a small saucepan and simmer on the stovetop over medium heat for 5 minutes until reduced.