Erwtensoep, or snert, is the soup that Dutch skaters drink from paper cups at the koek-en-zopie stalls on frozen lakes, and that families leave to simmer on a Saturday in January. A proper snert is so thick that a spoon should stand upright in the pan, and it tastes better on the second day. The traditional pot is full of pork and smoked sausage; this version gets its depth from celeriac, leek, a little smoked paprika and a long, slow simmer. Serve it with dark rye bread (roggebrood), butter and mustard. Chef's tip: stir the bottom of the pan every 10 minutes in the last half hour; split peas settle as they thicken and catch easily, and a scorched taste spreads through the whole pot.