Run your finger up and down the center of the salmon feeling for any pin bones. Remove any that you find with a needle-nosed pliers or tweezers. With a sharp knife cutting across the width of the salmon, divide it into 4 equal portions. Lightly salt the salmon and let sit a couple minutes; this will help you get crispy skin.
Heat a 2-count, about 2 tablespoons, of olive oil in a large skillet over medium-high heat until the oil is almost smoking. Season the salmon with pepper, and rub about 1 1/2 tablespoons butter on the skin side of each fillet. Add the salmon to the pan, skin-side down. To get super crispy skin, cook the salmon almost to completion, about 6 minutes.
Flip the salmon and cook until flesh side is nicely seared, 1 to 2 minutes.
Transfer the salmon to a platter and serve with the Smashed Potatoes.
Put the potatoes into a large pot, cover them with cold water, and add a large pinch of salt. If they're large, cut them in half. Bring to a boil and simmer until the potatoes are fork tender, 20 to 30 minutes. Drain. Stick a fork into the potatoes, 1 at a time, lift them out of the colander and peel with a paring knife. Toss the potatoes into a bowl and roughly crush them. Drizzle with olive oil and season with salt and pepper.
Heat 2 tablespoons olive oil with the butter in a medium saucepan over medium heat until the butter melts. Add the pearl onions, sugar, and lemon juice and cook, stirring frequently, until the onions are browned, 5 to 6 minutes. Add the lemon slices, peas, and lemon zest and continue cooking until the peas are hot. Season with salt and pepper. Dump the vegetables over the potatoes in the bowl, drizzle with extra-virgin olive oil add the parsley and dill and taste for salt and pepper. Scatter the watercress over the top, fold it in just until it wilts and call it a day.