Rinse the quinoa under cold water and set it aside in a small pot with 2 cups of water for later cooking.
Place the pork tenderloin in the slow cooker and season it with the salt and pepper.
Add the halved Brussels sprouts, minced garlic, chopped rosemary, lemon zest, lemon juice, chicken broth, and olive oil around the pork.
Cover and cook on low for 5 to 6 hours until the pork reaches an internal temperature of 145 degrees.
About 15 minutes before the pork finishes, bring the quinoa to a boil, then reduce to a simmer and cook for 12 to 15 minutes until the water is absorbed and the quinoa is fluffy.
Remove the pork from the slow cooker and let it rest for 5 minutes before slicing.
Serve the sliced pork with the Brussels sprouts and cooking juices spooned over the quinoa.