PREHEAT a grill to medium-high heat (375°F) or a grill pan over medium-high heat.
GRILL THE STEAK AND ONIONS: Season the steak with the cumin seeds, salt, and pepper. Cut the onion into ½-inch-thick slices. Drizzle the steak and onions with the olive oil.
GRILL the steak and onion slices over direct heat, turning once, until the onion is lightly charred, 5 to 6 minutes, and the steak is cooked to desired doneness, 15 to 20 minutes for medium (160°F). Remove the steak and onion and let rest for 5 minutes.
MAKE THE DRESSING: Meanwhile, in a small bowl, combine the mayonnaise, lime zest and juice, and hot sauce.
THINLY slice the steak against the grain and coarsely chop the onions. Place the lettuce in a serving bowl and top with the steak and onions. Drizzle the dressing over the salad. Top with the avocado and cilantro.