For the mushroom topping, heat oil in a pan. Saute chopped garlic and onions. When the onions are slightly brown, add the blanched and chopped mushrooms, chopped red, green, and yellow bell peppers, chopped cherry tomatoes, red chilli flakes, rosemary leaves, chopped parsley, black pepper powder, and salt. Mix well and cook for some time. The mushroom topping is ready. Halve the idlis horizontally. Heat butter in a pan. Toast idlis once butter melts. Set aside. Place lettuce and rocket leaves on each toasted idli. Add the mushroom topping on the leaves. Cut strips of cheese slices and add them on the mushroom topping. Heat idlis with topping in a microwave until cheese melts. Transfer the idlis to a serving plate. Garnish with some chopped parsley.