In a bowl, combine the olive oil and lime juice and season with salt and pepper. Add the avocados and red onions to the bowl and gently coat with the lime dressing. Set aside.
Prepare a grill (or griddle) for medium-high heat. Grill the pineapple slices until grill marks appear, about 2 minutes each side.
To assemble the salad, start with the watercress, then alternate the avocado and red onion mixture with the grilled pineapple. Drizzle the remaining dressing over the salad. Sprinkle the cilantro leaves on top.