• 5 tbsp. butter, divided, plus more room temperature for brushing
  • 2 large yellow onions, thinly sliced
  • Kosher salt
  • 8 oz. cavatappi or other short curly pasta
  • 2 tbsp. all-purpose flour
  • 2 1/2 c. whole milk
  • 1 c. grated fontina cheese
  • 2 1/2 c. grated Gruyère or cheese of your choice
  • 1 c. shredded sharp white cheddar
  • 1 tsp. Dijon mustard
  • Pinch of cayenne pepper
  • 1 tsp. fresh thyme leaves, plus more for garnish