Have ready a 1-qt (1-L) terrine mold. For ease of unmolding, line with plastic film.
Trim the stems from the spinach and wash it well in several changes of water. Cook in boiling, salted water until done, about 2 minutes, and drain. Rinse under cold water to cool. Drain. Squeeze dry.
Chop the spinach into fine pieces by hand or in a food processor.