1. Wash the baby portabella mushrooms under cold running water and clean them properly.
2. Pat them dry and thinly slice them.

Caramelizing the onions

1. Heat some olive oil in the pan.
2. Cut the onions into rings and add it to the pan. Let them sweat in the pan for around 10 minutes. Keep the heat to the medium .
3. Sprinkle salt and sugar over it. It will help in the caramelization process.
4. Keep stirring the onions from time to time. If it is drying out quickly then keep it moist by sprinkling some water over it.
5. After 20 minutes or so lower the flame and let the onions stick to the bottom a little. But before it burns out, stir them. Continue this process till you get the nice caramelized onions.
6. So make it ahead and save some time. It will take around 30 minutes.

Making the mushroom melt

1. In another pan heat some olive oil and once the oil starts simmering add the sliced mushrooms.
2. Let it sweat for few more minutes.
3. Once the mushrooms starts changing their color, add the black pepper powder.
4. Take care not to burn the mushrooms.
5. Deglaze the pan with balsamic vinegar.
6. Let the mushroom soak the flavour for another 5 minute.
7. Check if it needed extra salt or not. Mushrooms will get fully cooked in 10 minute straight.
8. Meanwhile warm the buns on griddle.
9. While the mushrooms are still warm, separate it into serving portion and top it with the cheddar slices.
10. Go for extra sharp cheddar if you can find it.
11. Let the cheese melts on top of the mushrooms.
12. Arrange the buns on a serving plate and arrange the greens over it.
13. Arrange the sandwich in this order – base bun followed by greens, top it with mushroom melt followed by sweet caramelized onions and a dash of Sriracha sauce.
14. Serve immediately.