Rinse pork roast under cold water, pat dry with paper towels.
Heat oil in a large pan. Brown roast on all sides, until golden brown. Remove roast from the pot and transfer to a slow cooker.
Cook diced onions in the same pan until translucent, scraping up all the browned bits in the pan.
Transfer the onions to the slow cooker.
Add chicken broth to pot to deglaze. Pour broth over browned roast in slow cooker. Add water to make sure the roast is covered at least ⅔ if needed.
Add whole cloves, bay leaves to the slow cooker.
Cover and cook on low for 8-9 hours or on high for 5-6 hours; until the meat is tender and easily comes off the bone.
Remove roast from slow cooker and discard bone. Keep roast warm.
Fish out and remove bay leaves and cloves from the juices in the slow cooker. Discard.
In a small bowl mix together cornstarch and water. Add cornstarch slurry to the juices in the slow cooker.
Place sour cream in a small mixing bowl. Add about a ½ cup of the juices from the slow cooker to temper the sour cream. Whisk until sour cream is well combined with the juices and no lumps remain.
Add sour cream mixture to the slow cooker.
Use an immersion blender to purée sauce in the slow cooker until smooth, well combined, and creamy.
Return pork to the slow cooker, cover and cook on high until sauce thickens, 20-30 minutes.
To serve place pork on a bed of mashed potatoes and top with gravy.