Melt the butter in a large skillet over medium heat and add the chopped onions and salt. Toss to coat and cook until slightly softened, about 10 minutes. Reduce the heat to low, and continue to cook until a deep caramelized color, about 45 minutes. Add the brown sugar and cook another 15 minutes, moving the onions frequently around the skillet.
Remove the leaves from the Brussels sprouts or slice, using a food processor fitted with the slicer attachment. Rinse and place the leaves in a microwave-safe bowl. Cover with plastic wrap and microwave for 35 seconds. (Alternatively, steam the leaves over boiling water for 3 - 5 minutes.)
Whisk together all of the ingredients for the Maple Dressing except the oil, until the salt dissolves. Slowly whisk in the oil.
Melt the butter for the Garlic Breadcrumbs in a medium skillet over medium heat and add the garlic and sliced scallions. Sauté until soft and add the breadcrumbs. Toss to coat and sauté until just starting to crisp. Stir in the dried cherries and pepitas and continue to cook until the breadcrumbs are crispy.
The caramelized onions, Maple Dressing, and Garlic Breadcrumbs can all be made ahead, allowing this dish to be whipped together in about 15 minutes for dinner.