Remove the stem from the portobello. If necessary, clean them with damp paper towel or brush them with a pastry brush. NEVER rinse mushrooms with water. Place them "cap" side down.
Distribute the garlic and butter in the mushrooms. Top with a halved cherry tomato and crumbled feta cheese. Add some muscovado sugar on each tomato halfe.
Add some fresh thyme on top and drizzle a teaspoon of olive oil over each mushroom.
Grill in the oven for 8-10 minutes.
Serve with arugula (rucola), and some extra thyme. Drizzle some balsamic glaze and add salt if required.