Escarole has a slight, pleasant bitterness balanced by a sweetness. It turns mild, even sweeter, and surprisingly buttery when cooked.
Thinly slice off both ends of one lemon. Cut into 8 thin slices. From remaining lemon, squeeze 1 to 2 tablespoons juice into a bowl.
In a 5-quart Dutch oven or heavy pot, heat oil over medium-high. Cook onion and garlic, stirring occasionally, until golden brown, 6 to 8 minutes.
Stir in escarole and 1/2 cup water. Arrange salmon on top; season with salt and pepper. Place 2 lemon slices on each fillet. Cover, and cook until salmon is opaque throughout, 12 to 14 minutes.
Transfer salmon to a plate. Stir lemon juice into escarole mixture. Serve salmon with escarole.