Heat the olive oil in a non-stick frying pan then add the sliced onion and cook on a medium heat until completely softened, so around 15 mins, adding a little splash of water if they start to stick at all. Crush in the garlic and cook for a 2-3 mins further, then add the butter. Once the butter is foaming a little, add the mushrooms and cook for around 5 mins until completely softened. Season everything well, then tip onto a plate.