The secret to a good French onion soup is the long slow cooking of the onions. You slice them then you cook them in butter at a low heat until the onions are really soft and just beginning to caramelize. That’s what gives French onion soup its incomparable flavor.
This soup starts with something like that. Onion, thinly sliced, cooked long and slow in butter until melt-in-the-mouth soft and lightly browned. When the onions had reached that stage, I poured in the broth, the cubed cooked meat and the squash. Ten minutes later, the spinach leaves went into the pot. Two minutes after that, it became this gorgeous soup. »