Heat a splash of olive oil in a pan over a medium heat. Finely chop 1 onion, then sweat it in the pan for 5 minutes with the lid on. Add 250g sliced chestnut mushrooms and 1 tsp fresh lemon thyme leaves, then cook for 5 minutes until the onions are golden and the liquid has evaporated. Add 100ml crème fraîche, then bring to a simmer. Stir in 200g baby spinach and let it wilt.
Meanwhile, cook 4 x 150g rump steaks in a griddle over a high heat. Cook for 1½ minutes on each side for medium-rare, or longer if you prefer. Serve with the mushrooms and boiled baby potatoes