How to Make It
Bring a large pot of salted water to a boil. Tie the leeks together with kitchen string, using 6 halves per bundle. Add the bundles to the boiling water and cook until just tender throughout, 10 to 15 minutes. Drain and transfer the leeks to a large bowl of ice water to cool completely. Drain again and pat dry; cut the strings.
Line an 8-by-4-inch loaf pan with plastic wrap, allowing several inches of overhang on each side. Arrange the leeks in the pan, cut side up, overlapping them slightly; season each layer with salt and pepper. Cover with the overhanging plastic wrap. Set another small loaf pan on top of the leeks, weigh it down with cans and refrigerate overnight.
In a bowl, mix the crème fraîche with the lemon juice until smooth. Spoon the cream onto 8 small plates and mound a little caviar alongside. Uncover the leek terrine and pour off any liquid. Carefully turn the terrine out onto a cutting board and discard the plastic wrap. Using a serrated knife, cut the terrine into 1-inch slices. Set each slice on a plate and serve.