Step 1

Preheat oven to 350°. Toast walnuts on a rimmed baking sheet, tossing once, until golden brown, 8–10 minutes. Transfer to a medium bowl. Finely grate garlic over warm walnuts and toss with 6 Tbsp. oil.

Step 2

Heat a large cast-iron skillet over medium-high. Season steaks with kosher salt and black pepper; sprinkle all over with Aleppo-style pepper. Rub with 2 Tbsp. oil and cook, turning occasionally, until deeply browned and an instant-read thermometer inserted into the thickest part registers 130° for medium-rare, 10–12 minutes. Transfer steaks to a cutting board.

Step 3

Wipe out skillet and reduce heat to medium. Place scallions and 1 Tbsp. oil in skillet, season with kosher salt, and cook, turning occasionally, until softened and deeply charred, about 4 minutes. Transfer to cutting board; trim and cut into 1" pieces. Add to bowl with walnut mixture along with vinegar, cornichons, capers, mustard, thyme, and sugar and toss to combine; season with kosher salt and black pepper.

Step 4

Slice steak against the grain and sprinkle with sea salt. Serve with scallion sauce and more Aleppo-style pepper.