First mix together the rosemary, garlic, salt, and pepper. Then, rub it on the roast and set aside.
Spray the slow cooker with non stick cooking spray, then place the potatoes on the bottom. Place the onion slices over the potatoes and add the butter as well. Next, pour in the balsamic vinaigrette.
Put the roast on top of the potatoes and then arrange the orange slices on top of the roast and squeeze or pour the juice from one of the oranges over the entire dish.
Cover and cook on low for 8 hours.
Remove the roast and set aside letting it cool. Use a slotted spoon to remove the potatoes and set them aside.
Pour in the baby spinach into the juices and let it cook until wilted. This only takes a few minutes.
Remove the spinach with a slotted spoon and place in a bowl or on a plate.
Now, arrange the potatoes, roast, and spinach to serve.