Wipe out skillet with paper towel. Add butter and onion; cook and stir over medium-high heat 5 minutes. Stir in corned beef and potatoes; mix well. Press down mixture with spatula to flatten into compact layer.
Reduce heat to low. Cook 10 to 15 minutes. Turn mixture with spatula; pat down and continue cooking 10 to 15 minutes or until bottom is well browned. Serve immediately.