Put the onion slices into the slow cooker, and toss with the olive oil and salt. Turn the slow cooker on LOW and cook until the onions are very soft, brown, and reduced, about 10 hours. Turn off the slow cooker, remove the lid, and allow the onions to cool completely. Put a colander over a medium bowl and pour the onions into the colander, allowing all of the liquid to drain out. Reserve the liquid for soups. Transfer the drained onions to the 2-quart saucepan and stir in the chardonnay or balsamic vinegar. Cook the onions uncovered over medium heat until the liquid reduces to almost dry, about 20 minutes. Turn off the heat and allow to cool.
Preheat the oven to 350° F/175° C.
Slice the polenta into 32 pieces, about 16 per roll. Lay the slices on the parchment-lined baking sheets and brush with a little olive oil. Bake in the preheated oven about 15 minutes. Turn each piece and brush with more olive oil, then return to the oven until the edges start to brown, about 15 more minutes. Remove from the oven and transfer to a plate lined with paper towel to drain and cool.
To assemble, place a thin slice of goat cheese on each piece of polenta. Arrange some onion pieces on top of the cheese, then add a mound of the port cranberries.