Put the juniper berries, cloves, allspice and fennel seeds in a spice grinder and grind into a powder.
Mix the ground spices with the salt and sugar and then sprinkle evenly all over the spare ribs. Refrigerate for at least 2 days.
When you're ready to make the Pot Au Feu, put the onions cut-side up under a broiler. Broil on high until the onions are charred black on one side.
Wash the cured spareribs, to remove excess salt and the curing spices and place them in a large dutch oven in a single layer. Add the charred onion, along with the peppercorns, bay leaf, and water.
Bring the pot to a boil over high heat and skim any foam that rises to the top. Repeat until there is no more foam.
Turn down the heat to maintain a gentle simmer and then cover and simmer until the ribs are almost tender (about 1 1/2 hours).
Transfer the meat to a bowl and set aside. Strain the stock through a fine mesh, pressing on the solids.
Wash out the pot, and return the meat and stock to it along with the carrots. Cook for another 30 minutes.
Add the potatoes and cabbage and continue to cook until the potatoes are tender (an additional 15-20 minutes).
Serve the pot au feu with crusty bread like a baguette or bâtard.