Split Pea Soup with Marinated Red Onions

Cut celery stalks into 4-inch lengths; tie together firmly with string. In large stockpot or Dutch oven, combine 12 cups water, celery bundle, ham hock, bay leaves and thyme; bring to boil, skimming off any foam. Cover, reduce heat and simmer until meat begins to lose shape, about 1 hour. Remove celery bundle, bay leaves and thyme sprigs.

Pour split peas into large sieve. Rinse under running water. Add to pot; bring to boil, skimming off any foam. Cover, reduce heat and simmer until peas are almost tender and cooking liquid begins to thicken and look cloudy, about 40 minutes. Remove ham hock and place on cutting board; let cool. Remove and discard any skin, bone and fat. Cut lean meat into small bite-size pieces; set aside.

Meanwhile, in large skillet, heat oil over medium heat; cook garlic, celery, carrots, onion, potato, salt and pepper, stirring and scraping up brown bits from bottom of skillet, until vegetables begin to soften, about 8 minutes. Add to pot; bring to boil. Reduce heat, cover and simmer until vegetables are tender and peas begin to break down and smooth out, about 20 minutes. Add meat; simmer for 5 minutes. Taste and adjust seasoning as necessary.

MARINATED RED ONION: Meanwhile, cut onion from top to root end into paper-thin slices. Toss with vinegar and parsley. (Make-ahead: Cover and marinate in refrigerator for up to 1 day.)

Ladle soup into warmed bowls. Top each with marinated onions.