Drizzle a very large non-stick frying pan with cooking oil and fry the steaks over a high heat for 3 minutes on each side, or until cooked to your liking. Transfer to a plate, cover loosely with kitchen foil and set aside to rest.
Add the mushrooms to the pan and stir-fry for 5-6 minutes until golden, then add a quarter of the garlic and 2 tbsps mustard and cook for another minute. Stir in 150ml of the stock and all the crème fraiche, then add the lemon juice and season to taste with freshly ground black pepper. Keep warm over a low heat.
Meanwhile, coat a large non-stick pan with cooking spray and cook the remaining garlic over a medium heat for 1 minute. Stir in the cannellini beans and the remaining stock and cook for 2-3 minutes until heated through. Stir in the parsley, lemon zest and the remaining wholegrain mustard. Season to taste and roughly mash with a potato masher.
Cook the green beans in a pan of boiling water for 4-5 minutes until tender, then drain. Divide the mash and beans between plates, top the mash with the steaks and serve with the mushroom sauce spooned over.