Place the Cotechino in a pot filled with cold water, pierce it with a toothpick, and let it boil for 2 hours; with a pressure cooker, 40 minutes from the whistle will be enough.
When cooked, take it out and let it cool.
In a different pot, with only little water, boil the spinach and then cook them in a pan with a drop of oil and a garlic clove. When cooked, let them cool too.
Unroll the puff pastry, pierce it with a fork; place the spinach on it, being careful to lay them down evenly, add the Cotechino (without the casing), and roll everything up.