Quick-pickle the onions: Place sliced red onion in a small bowl with 1–2 tablespoons red wine vinegar. Let sit for at least 30–60 minutes. Drain before using.
Make the dressing: In a large bowl, whisk together lime juice and 1 tablespoon red wine vinegar. Slowly whisk in oil until combined.
Prep the produce: Dice mango, avocado, and cucumber into evenly sized pieces.
Assemble the salad: Add mango, avocado, cucumber, and drained onions to the bowl with dressing.
Toss and serve: Gently toss to coat. Season with salt and pepper. Serve immediately.
Notes
Use firm-ripe mango and avocado for clean cuts
Add avocado just before serving
Chill ingredients ahead for a colder salad
For a salsa-style version, dice everything smaller