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. You'll know it's ready to flip once it releases from the pan. It sticks because the crust is still forming, so don't nudge it around if it's not releasing. The protein will almost always tell you when it's ready. The multi-sear and rest method builds crust slowly while keeping the inside perfectly even, with no grey band and no overcooked edge.

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