Caramelizing onions or shallots significantly reduces the volume by about half, so plan accordingly.
Sweet yellow onions or shallots
Remove top and bottom off onion or shallot along with the papery skin. Slice into thin half-rounds. Over medium heat, melt butter in a heavy skillet and add onions or shallots; stir to coat well. Cover and cook for 15 minutes until tender. Reduce heat to low and continue to cook uncovered for 25 minutes, stirring occasionally to allow for even browning. Remove from heat and stir in 1-2 tablespoons of water or white wine to deglaze pan.