A crowded pan is why the mushrooms came out grey. Browning is the Maillard reaction, named for Louis-Camille Maillard, who described it in 1912: sugars and amino acids breaking apart and recombining into hundreds of new flavour molecules. It only gets going in earnest somewhere above 140 degrees Celsius. But a wet surface can’t get hotter than the water on it, and water boils off at 100. So everything depends on getting rid of the water faster than the food gives it up. Spread out, the steam escapes, the surface dries, and the temperature climbs past the threshold. Piled in, the pieces steam each other, and the pan spends all its heat boiling water at a hundred degrees while nothing browns. Mushrooms, which are about nine-tenths water, make the point better than anything. Patting things dry and cooking in batches sounds like fussiness, but it is really just arithmetic about where the heat goes.