Heat a grill to medium heat. Brush both sides of the pineapple slabs with coconut oil and sprinkle with salt and pepper. Grill for a few minutes on each side until you have nice char marks. Let cool and chop into bite size pieces.
To prepare the dressing: place the mint leaves, garlic, diced onions, lime juice, honey, salt and pepper in a blender or food processor. Begin to blend and with the motor running, adding the olive oil in a slow stream. Taste for seasoning.
Add the romaine and watercress to a bowl or serving platter and drizzle with some of the dressing. Toss to coat. Top with pineapple, red onion, avocado slices and crushed plantain chips. Drizzle with a little more dressing and serve. Take care not to overdress the salad.