Pat the sirloin steak dry with a paper towel and season both sides with half of the sea salt and black pepper.
Heat the avocado oil in a cast-iron skillet over medium-high heat until the pan is shimmering.
Place the steak in the hot skillet and sear for 3-4 minutes per side for medium-rare, or until it reaches your desired doneness.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes to keep the juices intact.
While the steak rests, peel the sweet plantain and slice it into 0.5-inch thick rounds.
Add the plantain slices to the same skillet and sauté for 2-3 minutes per side until they are caramelized and tender.
In a medium bowl, combine the mixed baby greens, diced avocado, minced red onion, and lime juice.
Toss the salad gently with the remaining sea salt and black pepper.
Slice the rested steak against the grain into thin strips.
Plate the steak alongside the caramelized plantains and the avocado salad, then garnish with fresh cilantro.