Prepare the watercress: Place the watercress in a colander and rinse under cold water to remove and grit or sand. Discard any yellowing leaves and remove the tough stems; for this salad, you want only the leaves and tender stems. Pat dry with paper towels and refrigerate while you prepare the rest of the salad.
Broil the pineapple: Heat the broiler. Place the pineapple slices on a baking pan and sprinkle sugar evenly on top of them. Broil about 4 inches from the heat source, turning once, for 5 to 10 minutes on each side, until golden brown. (You can line your pan with aluminum foil for easier cleanup, but not parchment—it may catch fire.) Let cool, then cut into 1-inch cubes. Set aside.
Prepare the dressing: Place the garlic, olive oil, vinegar or lime juice, cumin, salt, and pepper in a small bowl and whisk to combine. Taste for seasoning. Set aside.
Peel the avocado: Cut the avocado(s) lengthwise in half around the pit and remove the pit. Place the avocado halves cut side down on the work surface and slice lengthwise into 1-inch-wide wedges. Peel each segment by hand or with a paring knife, and cut into 1-inch cubes.
Assemble the salad: Place the watercress in a medium bowl and toss with half the dressing. Arrange on a large platter.
Add the pineapple and avocado to the same bowl and toss with the rest of the dressing. Mound the pineapple and avocado over the bed of watercress. Garnish with slivers of red onion and serve immediately.
Add the pineapple and avocado to the same bowl and toss with the rest of the dressing. Mound the pineapple and avocado over the bed of watercress. Garnish with the slivers of red onion and serve immediately.