Place onions in bottom of large pot.
Season roast well with salt and pepper, and place it on top of the onions.
Pour vinegar around the bottom of the roast (no additional liquid will be needed - the roast will make its own gravy.)
Heat over high heat to simmering, then reduce heat to low and cook 3-4 hours covered (3 hours was plenty for ours), without taking the lid off while it's cooking.
Remove roast from pan.
Whisk together flour and water.
Pour flour mixture into drippings on the skillet ~ stir until thickened.
Carve roast and serve with gravy.
Enjoy!