Pat the sirloin steak dry with paper towels and cut into 1-inch cubes.
In a small bowl, whisk together the lime juice, fish sauce, minced garlic, and black pepper to create the vinaigrette.
Arrange the watercress, halved cherry tomatoes, and sliced red onion on a large plate.
Heat the avocado oil in a large cast-iron skillet over high heat until it begins to shimmer.
Add the beef cubes in a single layer and sear without moving for 2 minutes to develop a dark, savory crust.
Pour the coconut aminos into the pan and shake the skillet vigorously for 1 minute to glaze the beef.
Transfer the hot beef and pan juices onto the watercress salad and drizzle with the lime vinaigrette.
Season with sea salt and serve immediately while the beef is warm and the greens are fresh.