Step 1Combine the milk, garlic cloves and ginger in a heavy medium saucepan. Tie all but 12 small, tender sprigs of the thyme into a bundle with kitchen twine. Add the thyme bundle to the milk mixture. Bring to a simmer. Simmer gently, uncovered, over low heat until the garlic is very tender and the liquid is reduced by one-fourth, stirring occasionally, about 23 minutes. Discard the thyme bundle.
Step 2Working in batches, puree the milk mixture in a blender until smooth. Return the soup to the saucepan. If necessary, simmer the soup over low heat until it is reduced to 3 1/2 cups. Season the soup to taste with fleur de sel and pepper. (At this point the soup can be prepared up to 8 hours ahead. Cover and refrigerate. Rewarm before serving.)
Step 3Melt the butter and oil in a heavy, large saute pan over high heat. Sprinkle the scallops with fleur de sel and pepper. Working in batches, add the scallops to the pan and sear until golden brown and just cooked through, about 2 minutes per side.
Step 4Mound the cress in the center of each of the 6 wide shallow soup bowls. Place 2 scallops atop the cress in each bowl. Surround the cress with the reserved small thyme sprigs. Tear the gold leaf, if using, into large nonuniform strips and arrange it around the scallops; the gold leaf is delicate and will tear easily. Ladle enough soup into the bowls to come halfway up the sides of the scallops.