Saut� the onion in a skillet with a little oil till tender. Add to the crockpot with the potatoes, carrots, rosemary, broth and parsley. Add water to about 2� from the top of the crock, season with salt and pepper and cook on HIGH for 8 hours.
An hour before serving put milk into a measuring cup or jar, add flour and whisk till combined, add to the soup.