Spring Leek Soup with Gruyere Toasts

butter

large leeks (white and light green parts, sliced in half longwise, rinsed and thinly sliced crosswise)

fine sea salt

chicken stock

ground white pepper

Meyer Lemon (juice and zest)

, 1/2- inch slices crusty bread (toasted (I used whole wheat levain))

Gruyere cheese (shredded)

green onion (thinly sliced (for garnish))