Start by making the spaetzle, which can be made up to a day ahead. Mix all the ingredients together in a large bowl; it should look like really thick pancake batter, not a dough. Set a large pot of salted water to a boil. By the time the water boils, the batter will be ready. I use a spaetzle maker to make my spaetzle, but you could use a colander with large holes, too. Get a bowl of ice water ready, and a cookie sheet with a little cooking oil in it.
Load up the spaetzle maker with the batter and use the hopper to make the dumplings. Let them boil for 1 minute after they rise to the surface. Move the dumplings to the ice water and let them cool off for a minute or three, then move them to the cookie sheet. Toss them with the oil so they don't all clump together. Set aside.
While you are waiting for the spaetzle water to boil, heat the oven to 375°Toss the radish wedges in the melted duck fat and salt well. Put them uncovered in an ovenproof pan and roast until they are soft and caramelized on the edges, about 45 minutes. Remove and set aside.
When the radishes have about 15 minutes to go -- or you can just wait until they are done -- heat the duck fat for the goose in a large saute pan. Sear the goose breasts according to these directions, then let them rest on a cutting board, skin side up. Sprinkle the ground black trumpet mushrooms on the skin the moment you set them on the cutting board.
Add the minced onion and black trumpet mushrooms to the pan and turn the heat to high. Toss to coat with the duck fat (add more if you need tand sprinkle everything with salt. Let this cook, stirring often, until the mushrooms give up their water, about 6 to 8 minutes. Let them sizzle until the onions get a little browned at the edges.
The radishes should be done by now. Set the resting goose breasts in the still-warm oven. Don't turn it on.
Add the black garlic to the pan and squish it so it combines with the onion and mushrooms. Add the chopped kale and saute until it wilts. Pour in the stock and boil it down furiously until it thickens into a sauce. Add the spaetzle and rosemary to the pan, toss to combine and turn off the heat. Add the beer or malt vinegar to taste.
To serve, give everyone some of the contents of the pan, then some of the radishes. Slice the goose breast into three pieces and give everyone one full breast. Garnish with some of the myzithra cheese for "snow."