Decrease heat to medium. Add 2-3 tablespons of water, cover the pan with a lid and let simmer for 7-10 minutes (7 minutes for a bit firm sprouts and 10 for well cooked).
Increase heat to over medium - high. Add the Brussels sprouts and mushrooms and cook, stirring, for 5 minutes until they start to turn golden.
Add the leek and cook for 2-3 minutes until start to soften.
In a large pan heat the oil and butter to medium heat.