Dipping sauce:

• ¼ cup finely chopped sauerkraut, drained
• ¼ cup mayonnaise
• ¼ cup sour cream
• 2 Tbsp. chopped fresh parsley 
• 2 Tbsp. Stone-ground or dijon mustard
• ½ tsp. Kosher salt
• ½ tsp. Worcestershire sauce

Quesadillas:

• In a large saucepan over medium heat, heat oil.
• Cook onion and garlic, stirring occasionally, until softened about 5-10 minutes; season with 1 tablespoon salt.
• Reduce heat to medium. Add cabbage and cook, stirring occasionally, until wilted and caramelized, 5 to 6 minutes; season with remaining 1 teaspoon salt.
• Remove from heat and fold in corned beef and keep warm.
• In a medium nonstick skillet over medium heat, melt 1 teaspoon butter.
• Place 1 tortilla in a skillet, arrange some cheese on one side of the tortilla, then top with one quarter of cabbage mixture. Sprinkle cabbage mixture with a little more cheese.
• Fold the other side of the tortilla over to create a half-moon. Cook until the tortilla is toasted, 1 to 2 minutes per side. 
• Repeat with remaining butter, tortillas, cabbage mixture, and cheese.

• In a small bowl, combine sauerkraut, mayonnaise, sour cream, parsley, mustard, salt, and Worchestire.
• Cut quesadillas into halves or thirds. Serve with sauce.