Place morels in 2 cups boiling water; let stand 5 minutes. Remove morels with a slotted spoon, reserving broth. Strain broth, discarding any sandy residue in bottom of pan. Chop morels. Set morels and broth aside.
Heat butter and oil in a large, deep skillet over medium-high heat until butter melts. Add fresh mushrooms and onion. Cook, stirring constantly, 5 minutes or until onion is browned; add morels, wine, and vinegar. Reduce heat, and simmer, uncovered, 2 minutes. Add reserved broth; bring to a boil, reduce heat, and simmer, uncovered, 3 minutes.
Combine flour and water, stirring until smooth. Add to mushroom mixture. Cook, stirring constantly, 2 minutes or until thickened and smooth. Stir in half-and-half and remaining ingredients. Cook 5 minutes. Serve over Yukon Gold Mash.