Char the Peppers: Heat a skillet over medium heat with a drizzle of olive oil. Add Jimmy Nardello peppers and cook, turning occasionally, until blistered and lightly charred on all sides. Remove and let cool. Trim the stems.

Make the Vinaigrette: Add the charred peppers, balsamic vinegar, Dijon mustard, whole grain mustard, and olive oil to a wide-mouth jar or blender. Use an immersion blender (or regular blender) to blend until smooth. Season to taste with kosher salt.

Cook the Steak: Pat steak dry and season with kosher salt. Heat a skillet over medium-high heat. Sear the steak for 2 minutes per side, remove and let rest for 4 minutes. Repeat until the steak reaches 130°F internal temperature for medium-rare. Rest 5 minutes, then slice thinly against the grain.

Assemble the Salad: Toss arugula with a spoonful of vinaigrette. Arrange sliced steak on top and drizzle with additional vinaigrette. Finish with flaky salt.