Peel the onions. Cut them in half through the root-stem axis and then, working in the same direction, slice the halves into 0.5cm-or-so-thick slices—they should be thin but not too thin—with a knife or on a mandoline. You should have 2½ to 3 loosely packed cups sliced onions.
Combine the onions, butter, water and salt in a saucepan large enough to accommodate t