Thoroughly dry the fish and season it to taste with salt and pepper.

Heat a large non-stick pan over medium-high heat. Add the pine nuts and cook, stirring often, until golden and fragrant. Transfer to a bowl and set aside.

In the same pan, add the butter and when the butter is melted, add the fish. Cook until lightly browned but not cooked through, about 2 minutes per side. Transfer to a plate and set aside.

Add the onions. Season with a bit of salt and cook, stirring often until they are soft and translucent, about 10 minutes. Add the wine and cook until no liquid remains and the onions are golden, another 5 minutes.

Return the fish to the pan, lower the heat and cover. Cook on low until the fish is cooked through (note that if using sole, you will likely not need any further cooking and cooking times will vary depending on the thickness of the fish - you want to cook it until it just begins to flake when tested with a fork).

Off heat, stir in toasted pine nuts and parsley. Season to taste with salt and pepper, serve.