Chipotle peppers are smoked jalapenos. You can find them in adobo sauce in ethnic sections of major grocery stores. They add a complex heat when brushed on steak before grilling.
Sprinkle steaks lightly with salt and pepper. Spread each with 1 teaspoon adobo sauce.
For a charcoal grill, grill steaks on the rack of an uncovered grill directly over medium coals until desired doneness, turning once. Allow 10 to 12 minutes for medium rare (145 degreesF) or 12 to 15 minutes for medium doneness (160 degrees F). (For a gas grill, preheat grill. Reduce heat to medium. Place steaks on grill rack over heat. Cover; grill as above.)
Meanwhile, for dressing, in a screw-top jar combine the chopped pepper, olive oil, and vinegar. Shake to combine.
Slice steaks and arrange on serving plates with tomatoes and avocado slices. Top with onion slices and drizzle with dressing. Makes 4 servings.
Omit oil and vinegar and stir the chipotle pepper into 1/2 cup bottled ranch dressing for a creamy dressing.