Place potatoes in large pot and cover with cold water. Bring to a boil, season the water with salt and simmer until tender. Drain.
When potatoes are cool enough to handle but still very hot- peel them and return the flesh to the warm pot. Mash potatoes in the warm pot allowing them to release steam and dry out slightly.
Meanwhile, heat the cream in a saucepan until warm. Add the warm cream and butter to the potatoes.Stir through until completely smooth and then season well with salt. Roughly chop a handful of coriander and stir through the mash.
For the pea purée:
Bring a medium sized pot of water to the boil, and salt generously. Blanch the peas for 2-3 minutes until tender. Plunge peas into an ice bath briefly and drain. Add the peas to a blender with the lemon juice and butter. Blitz the peas, adding splashes of water in very small additions if needed. Blend into a smooth purée and then pass through a fine sieve. Season with salt.
For the steak:
Season steak all over with salt and pepper. Heat a large seasoned cast-iron pan over high heat. Brush steak generously with avocado oil and brush the pan with avocado oil. When the pan is smoking hot- cook the steak, turning every 2–3 minutes, until a dark crust forms on both sides and the steak is very rare.Reduce heat to medium- add butter, garlic, and rosemary. Baste steak with butter until done to your liking. Transfer steak to a board and it rest for 10-15 minutes.
For the mushrooms:
Wipe pan with a paper towel and add a fresh drizzle of olive oil. Cook portabellos with a few sprigs of thyme, salt and pepper on high heat until golden brown but still nice and juicy. Serve steak with golden brown mushrooms, mash and pea purée.