Melt the butter in a skillet over low heat. Add the sliced leeks and cook, covered, until the leeks are tender and lightly colored, about 30 minutes. Stir frequently or the leeks may scorch. Remove from the heat and cool slightly.
Whisk the eggs, yolks, half-and-half, and heavy cream together in a bowl and season with salt and pepper. Add a grating of nutmeg, i