Like all roasted vegetables, roasted mushrooms are pretty glorious. If you've ever made them, you've most likely noticed a bit of liquid pooling in the pan. This liquid should be drained to encourage browning, but it should not be tossed, as it makes a delicious, savoury condiment.
Serious Eats calls it "mushroom juice", but it's really mushroom juice plus olive oil, salt and pepper and whatever other seasonings you've tossed with your 'shrooms. Like mushrooms themselves, it's packed with umami goodness, and can be used just like soy sauce. Stir it into stews, drizzle it over rice or use it as a finishing sauce on meats, seafood and vegetables.