Use a tablespoon to scrape and remove the dark gills under the mushroom cap. Place the cleaned mushroom in a mixing bowl. Toss with the extra-virgin olive oil and half of the minced garlic and shallots. Transfer to a baking sheet and soak with balsamic vinegar. Roast in oven until just tender, about 15 minutes. Remove and let cool. Place spinach in a small sauté pan over medium heat with butter, remaining minced garlic and shallots, season with salt and pepper, and then cook about 1 minute. Place the cooked spinach in a small strainer and press with the back of a spoon to drain off any excess cooking juices. Reserve wilted spinach for the stack.

Begin cutting perfect cylinders with a ring mold from the center of all the sliced stacking ingredients; yellow tomato, red sweet pepper, mozzarella, and roasted portobello. Use a 3-inch ring mold or a soup can with both ends removed to form the stack. Build the stack on a small plate so it can be later transferred to the presentation plate.

Begin layering the stack by starting with mushroom cutout on bottom, and then stack the mozzarella, red sweet pepper, spinach, and yellow tomato, alternating ingredients. Top with a mushroom cutout with the top of the mushroom cap facing up. Hold in place by skewering with a fresh rosemary branch. Push firmly on top of vegetable stack to remove any excess cooking juices, then use a spatula to transfer entire ring mold to center of the presentation plate. Slide ring mold up and remove to expose the vegetable stack.