Preheat the pan, I used cast iron, add the oil and brown the roast over medium-high on all sides. Remove roast from skillet and place in the crock-pot. Place onions around the roast and sprinkle rosemary over the top. Pour the water in, then scatter the sea salt over the top of the roast. Turn crock-pot to high and cook for about 8-9 hours or until tender and falling apart.
When ready to serve place the roast on a serving platter and cover to keep warm. Transfer liquid and onions from the crock-pot to a pan on the stove. Bring to a boil then reduce liquid to a simmer.
Mix together the arrowroot and water together and add a little of the boiling liquid to the mixture to temper it. Then, slowly add the arrowroot mixture to the simmering liquid whisking all the while. Whisk in remaining salt and simmer for a few minutes until thickened. Transfer to a serving container and serve with the roast.