2 6-ounce fillets branzino or mackerel
2 tablespoons vegetable oil, divided
Mixed Olive Vinaigrette
Place fish on a plate, skin side up, and chill, uncovered for one hour. (This dries out the skin and is a crucial step.)
Heat a large stainless-steel skillet over high. Once the skillet is extremely hot, about 2 minutes, add 1 Tbsp. oil and a large pinch of salt. Heat oil until smoking, then remove skillet from heat and wipe out with a clean kitchen towel (this creates a temporary seal on the pan, like a faux nonstick surface).
Season both sides of fillets with salt. Return skillet to high heat and add remaining 1 Tbsp. oil. Once oil is hot and shimmering, add 1 fillet, lowering it into the pan away from you, skin side down. Press gently with a spatula until fillet no longer wants to curl up and entire skin side is flush with the skillet, ensuring complete and direct contact. Repeat with remaining fillet. Cook fish, pressing periodically with spatula, until fish is nearly cooked through (it should still be opaque and a bit raw at the thickest part of the fillet), about 3 minutes. Gently turn fish (working away from you so the oil doesn't splash on you) and finish cooking off heat, about another minute or so.
To serve, spoon a bit of vinaigrette onto plates and place fish on top, skin side up (no point in working so hard to create a beautifully crisp skin only to cover it up with sauce!).