First you want to add you potatoes to the bottom of your slow cooker. Then add carrots and lay thick onion slices on top. Add small cubes of butter and no need to stir. Butter will melt enhancing the flavor of au jus.
Slowly pour beef stock mixture on top of roast, making sure to get all the last bits that may have settled at the bottom of the bowl. Do not clean the roast or stir anything. Cover your crock pot with a lid.
A lean roast like sirloin tip roast benefits from slow cooking on low heat. I recommend to slow cook it for 8-10 hours on Low. You can absolutely cook it for 4-5 hours on High heat but it might be a bit more tough.