Heat a soup pot over medium heat; add the butter and melt
. When the butter foams, add the onions, leeks
, garlic, bay leaf, thyme and salt and pepper. Partially cover and cook, stirring occasionally, until caramelized, about 35 minutes. Deglaze
the pot with sherry
, then add the Worcestershire and stock. Taste for seasoning when it comes to a bubble. Cool and store for a make-ahead meal.
Heat the soup
and creme fraiche over medium heat until bubbly and thickened a bit. Toast the bread on the middle rack under the broiler; remove and top one side with the mustard and cheese. Return to the oven to melt the cheese.
Serve the soup in shallow bowls, topped with scallions
and cheesy croutons.