Rinse the mushrooms gently in a bowl of water, making sure to remove any grit from the heads and stems. Some of them may have hard woody stems. Cut these bits off and discard. Tear up any bigger mushrooms with your fingers; leave the smaller ones whole.
Lay the flat of your knife on the garlic clove and press down until it bruises lightly. Remove skin and discard.
Add oil to a pot and throw in the curry leaves and garlic. Once the curry leaves start to splutter, add mushrooms, salt and sambar powder. Cover and cook for 5-8 minutes until just tender. You want the mushrooms to still have a bite to them and remain intact.
Once cooked, turn the heat up to high and move the mushrooms around with a spatula until the cooking juices in the pot, if any, evaporate. (Don’t make it too dry; you want just enough moisture coating the mushrooms - see picture).
Add butter and toss in the residual heat. Serve hot or at room temperature. Best eaten with rice.
Notes
Use button mushrooms if you can't find oyster mushrooms.