Firstly, to prepare your mushrooms, they only need a wipe with a damp cloth. As mushrooms contain a lot of water, frying them over a low heat means they tend to release this liquid and just simmer in it, which is not ideal.  By using a medium-high heat, the moisture is absorbed as it's released, intensifying the earthy flavours and turning them almost a caramelised golden brown. Avoid overcrowding the pan too - fry in batches if necessary.