Combine lettuce, onion and pepper in a large bowl.
Whisk together ¼ cup olive oil, 1 tablespoon mustard and the red wine vinegar. Drizzle over lettuce, onion and pepper.
Smear 1 tablespoon of mustard and 1 tablespoon of olive oil over both sides of the steak. Salt the steak lightly and sear in a pan over very high heat, or grill on high. It should only take a few minutes on each side.
Remove from heat and slice thinly. Combine with lettuce. Add salt and pepper to taste.
