Put hocks, stock water, onion, and bay in large pot. Gently simmer; skim foam and partly cover. Cook 2 to 2 1/2 hours until meat tender and pulls from bones.
Lift hocks onto tray. Strain stock; discard onion and bay, skim excess fat, and measure 5 cups, adding water if needed.
Once cool to handle, discard skin, bones, and gristle; tear usable meat bite-size.
Heat butter and oil in clean soup pot over medium-high. Brown mushrooms 7 to 8 minutes; add garlic 30 seconds. Add wine; reduce by half, about 3 minutes.
Add measured stock, lentils, pearl onions, thyme, pepper, and meat. Simmer partly covered 30 to 40 minutes until lentils tender but intact and ham 165°F.
Add sherry vinegar; taste before adding up to measured salt. Divide among six bowls.