1 medium yellow onion, peeled and diced
3 to 4 large garlic cloves, peeled and minced
6 tablespoons butter, divided
4 cups beef broth
4 cups chicken broth
1 tablespoon Worcestershire sauce
1/2 teaspoon red-pepper flakes
4 medium Yukon Gold potatoes, peeled and cut into 1/2-inch dice
2 cups sauerkraut, drained and rinsed
3 cups (14 to 16 ounces) chopped pastrami (see note)
1/4 cup butter, softened
6 (1/2-inch-thick, or slightly thicker) slices good-quality rye bread
Russian Dressing (recipe follows)
2 cups shredded Jarlsberg cheese

In a large, heavy-bottomed pan over medium heat, saute the onion and garlic in 2 tablespoons of the butter until onion is soft and translucent. Add the beef and chicken broth, Worcestershire sauce, red-pepper flakes, potatoes and sauerkraut. Bring mixture to a boil, then reduce heat, cover and simmer for 10 minutes. Add the pastrami and continue simmering, covered, until potatoes are tender, for about 15 additional minutes.

While soup is simmering, prepare croutons. Spread some of remaining 1/4 cup butter on 1 side of each of the bread slices. Place slices, buttered-sides up, on a baking sheet and broil until well-toasted and golden-brown. Turn slices over and spread untoasted sides with more butter, then return to oven and broil until lightly golden-brown. Remove from oven. Trim croutons so each will fit within diameter of ovenproof bowls used for serving soup.

When ready to serve, ladle a generous portion of soup in each ovenproof bowl. Spread about 1 tablespoon of the Russian Dressing on each crouton and place it on top of soup. Layer a generous amount of the shredded cheese over crouton and surface of soup, then arrange all bowls on a baking sheet and broil until cheese has melted and turned a light golden-brown. Serve immediately. Makes 6 servings.

RUSSIAN DRESSING: In a small bowl, whisk together 3/4 cup good-quality mayonnaise, 1/3 cup chili sauce (similar to ketchup, only spicier), 2 tablespoons sour cream, 3 tablespoons finely chopped sweet onion, 2 tablespoons finely chopped garlic-dill pickle, 1 tablespoon fresh lemon juice, 1 tablespoon prepared horseradish (NOT cream-style) and 1/2 teaspoon Worcestershire sauce. Refrigerate until ready to use. May be made up to 7 days ahead. Makes about 11/2 cups.

NOTE: Even though a traditional Reuben sandwich uses corned beef, I prefer pastrami in this soup because it imparts a spicier and smokier flavor. If you prefer to use corned beef, that's OK! When purchasing pastrami at deli counters, have it sliced 1/2 inch thick (I usually end up with 4 slices measuring 1/2 inch thick). To prepare for soup, chop each 1/2-inch slice into 1/4-inch pieces (don't be precise; it looks better in soup if pastrami pieces are irregular in size and shape).