Cook the potatoes and garlic in a large pot of boiling salted water until they're fork tender, about 15 minutes. Strain. In a medium sized bowl, mash together the potatoes and garlic, butter, heavy cream, green onions, cheddar cheese, red wine vinegar, 1/4 teaspoon of sea salt, and freshly ground pepper to taste. Cover and set aside.
Add the fl our to a large mixing bowl and make a well in the center. Into the well add the egg, milk, and 1/4 teaspoon of sea salt. Using your hands, mix in the flour until all the liquid is absorbed and the dough sticks together. Shape the dough into a ball, wrap it in plastic wrap, and refrigerate for 20 minutes. Once the dough has chilled, dust a clean surface with fl our, and roll it out inch thick. Using the cookie cutter, cut out 12 circles.
On one half of each dough circle place 1 tablespoon of the potato mixture. Fold the other half of the circle over the potato mixture, and use a fork to press and seal the edges together. Lay out the pierogies on a large cookie sheet, and cover with a dish towel. In a medium pot, bring water to a boil, and drop in the pierogies 4 at a time. Boil uncovered for about 10 minutes, until they fl oat to the surface. Strain.
SHKIAFFING IT TOGETHER:
Mix the sour cream with the minced sweet gherkins and set aside. Heat the canola oil in a medium sized pan over medium-high heat. Add the yellow onion slices, and fry until they're dark and crisp, 3 to 5 minutes. Drain. Discard all but 2 tablespoons of the oil, and pan sear the pierogies in it for about 45 seconds per side, until they're lightly browned. Top the pierogies with the fried onion slivers and serve with the sweet gherkin sour cream.