Hanger Steak with Charred Scallion Sauce

Name: Hanger Steak with Charred Scallion Sauce
Ingredients: ½ cup crushed or coarsely chopped walnuts; 1 small garlic clove; 9 Tbsp. extra-virgin olive oil, divided; 1 1½-lb. hanger steak, cut into 4 pieces, center membrane removed; Kosher salt, freshly ground pepper; ¾ tsp. Aleppo-style pepper, plus more for serving; 12 scallions; 5 tsp. sherry vinegar or red wine vinegar; 1 Tbsp. chopped cornichons; 1 Tbsp. chopped drained capers; 1 Tbsp. whole grain mustard; ¾ tsp. chopped thyme; Pinch of sugar; Flaky sea salt