Quickly clean the mushrooms with water, remove the stems (they can be used for other recipes) and dry well.
Roughly chop the well-drained vegetable giardiniera from the conservation oil.
Cool the cotechino slices and, with the aid of a coppapasta with a 4 cm diameter, obtain 2/3 rounds from each slice.
Place on your work surface 12 champignon mushroom caps with the concave side up, place the spinach leaves on (previously, you should wash them and remove their stems), arrange the mini slices of warm Cotechino and the chopped giardiniera on top.
Arrange the remaining mushrooms with their concave side down; close the hamburger and put a toothpick in it to secure it.
Serve the mini hamburgers as a tasty finger food for your happy hour.