Season the tilapia with ½ teaspoon salt and ¼ teaspoon pepper. Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat.
Add half the tilapia and cook until opaque throughout, 1 to 2 minutes per side. Transfer to a plate. Repeat with 1 teaspoon oil and the remaining tilapia.
In a small bowl, whisk together the lime juice, honey, ginger, crushed red pepper, the remaining ¼ cup of oil, and ¼ teaspoon each salt and pepper.
Divide the tilapia, watercress, mango, and onion among plates. Drizzle with the dressing.
