completely tender but not falling apart, 8 to 10 minutes; a
cake tester or fork should pierce the potatoes with no
effort. Drain the potatoes, transfer to a large bowl, and set
aside.
Heat 4 tablespoons of the butter and the olive oil in a 12-
inch heavy-bottomed nonstick or cast-iron skillet over
medium-high heat until the butter melts. Add the onions
and cook, stirring frequently, until softened and
beginning to brown, 8 to 10 minutes. Using tongs or a
slotted spoon, transfer the onions to the bowl with the
potatoes, leaving the fat in the skillet. Fold the potatoes
and onions together and season to taste with salt and
pepper.
- Add 2 tablespoons butter to the skillet and heat over high
heat, swirling until the foaming subsides and the butter
starts to brown. Add the potato mixture to the skillet and
press down with a silicone spatula until an even cake is
formed. Reduce the heat to medium-high and cook,
shaking the pan gently every minute or so, until the
bottom is completely browned and verging on charred,
about 5 minutes. Use the spatula to lift and flip the
potatoes in sections, slicing and folding to incorporate the
browned sections into the interior. Press the cake into the
skillet again and repeat the browning and folding steps
two or more times: you should end up with a potato cake
that is riddled with crisp bits of potato and sweet onions. - Push the potatoes to one side, add the remaining 2
tablespoons butter, and melt it. Then lift the potatoes over
the butter and spread and press into an even layer. Cook
until the cake is completely crisp on the bottom, about 5