In a blender or using an immersion blender, combine the chipotle peppers with their sauce, oil, onion, garlic cloves, salt, pepper, garlic powder, paprika, and cumin. Blend until smooth and fully combined.
Add the steak and marinade to a bowl or bag and mix to coat the steak. Cover and marinate for at least 4 hours, or overnight for deeper flavor. (When ready to cook, let the steak sit at room temperature for 25–30 minutes.)
Heat a large skillet over high heat. Once hot, drizzle in a little oil and heat until it shimmers.
Add the marinated steak to the hot skillet. (Be sure to remove any excess marinate from the steak.)
Sear the steak undisturbed for about 4–5 minutes to develop a nice char. Flip and cook for another 3–4 minutes, or until it reaches your desired doneness. (Use a meat thermometer if needed: about 130°F for medium-rare, 140°F for medium.)
Once the steak is done, transfer to a cutting board and let the steak rest for 5-10 minutes.
Slice or cube the steak against the grain, then finish with flaky salt and a squeeze of fresh lemon juice.