Prepare the pickled red onions and the balsamic vinaigrette according to their recipes, then cover and chill until needed.
Preheat the grill to high heat (450–550°F / 230–290°C).
Coat the watermelon slices lightly if needed to prevent sticking, then grill each slice 20–30 seconds per side until grill marks form; transfer to the refrigerator to chill for about 20–30 minutes before serving.
Rub the flank steak all over with the tablespoon of olive oil and season generously with sea salt and fresh cracked pepper.
Grill the steak over high heat 3–4 minutes per side for medium-rare, using tongs to flip; remove from the grill and let rest on a cutting board for 2–3 minutes.
Thinly slice the rested steak on a bias into bite-sized strips.
Prepare the salad by combining the packed spring mix, halved cherry tomatoes, sliced cucumber, chilled grilled watermelon, thinly sliced radishes, sliced green onions, crumbled feta, and pickled red onions in a large bowl or on a platter; toss gently to combine.
Arrange the sliced steak over the salad, season lightly with additional salt and pepper if desired, and serve with the balsamic vinaigrette on the side.