The grilled pineapple in these stacks offers a smoky sweetness, and the portobellos are surprisingly high in healthy antioxidants. To make your stack look like a burger, double down on portobellos and cap each stack with another mushroom.
Per serving:
In bowl, whisk together marinade ingredients.
Grease barbecue grill with oil and preheat to medium high. On rimmed baking sheet, place mushrooms, yam slices and pineapple slices in single layer. Brush each item on both sides with marinade. Then dust one side of each pineapple slice with a pinch of smoked paprika.
Remove mushrooms, yam and pineapple from baking sheet and place them on preheated grill. Grill pineapple slices for 2 to 3 minutes per side. Grill mushrooms and yam slices for 3 to 4 minutes per side, or until done as you like. Return items to baking sheet as they are done to your liking.
To assemble and serve, place mushroom caps, stem-side up, on each of 6 serving plates. Spoon equal amounts of guacamole into each cap. Then top with slices of yam, pineapple and tomato. Season with salt and pepper, and spear each stack with a rosemary sprig. Serve warm.