Heat butter in a cast iron or other heavy-bottomed skillet over medium heat until melted and shimmering. Add the sliced onions, separating them with the spatula as you stir to coat them evenly in butter. Reduce heat to medium-low and allow to slowly carmelize, stirring occasionally for 20-30 minutes. Add more butter as needed if the pan begins to dry out. When the onions are soft and perfectly golden, add the maple syrup to the pan and stir a few times to combine. Remove from heat and allow to cool slightly. Meanwhile, split the baguette in half so that it opens into two long halves. drizzle a generous amount of olive oil across the interior surfaces of the baguette and lightly toast it in the oven. Remove from the oven and spread each surface with pâté. Top each with the jammy maple onions, sprinkle with fresh rosemary or thyme, and serve open-faced with a side of cornichons.