The Steinitz countergambit. Literally move 2. It's baffling to me why more people don't play it, it's one of the most popular responses even at the GM level and one of the best-scoring as well.
White doesn't get his usual slow game at all, black bolting out of the blocks like lightning.
If it was the case that it was unsound then sure, that would be a reason not to use it. It's perfectly sound and very good, I would never play anything else.
Is there a way in which Black can counter the London and prevent white from getting a solid position (effectively)?