Spread the diced onions and pickles into the bottom of the slow cooker.
Heat the oil in a large saute pan over medium-high heat.
Pat the roast dry with paper towels then generously season with salt and pepper.
When the oil is hot, sear the roast until well browned, about 3 minutes per side. Transfer roast into the slow cooker on top of the onions and pickles.
Add the garlic to the saute pan and cook while stirring until fragrant, about 5 seconds. Add the pickle juice. Using a wooden spoon scrape the bottom of the pan to release any stuck on bits then pour the juice into the slow cooker over top of the roast.
Cover the slow cooker and cook on low for 7 to 8 hours or until roast is tender.