Heat vegetable oil in a deep heavy-based pan until a breadcrumb sizzles.
Season plain flour with salt and black pepper.
Coat shallot rings in flour, dip in milk, and dredge in flour again.
Deep-fry shallots for 3-4 minutes until golden-brown and drain on kitchen paper.
Heat a griddle pan or barbecue to a high heat.
Season steaks and cook for 3-4 minutes per side, then rest for 5 minutes.
Sauté mushrooms in olive oil for 2-3 minutes until softened.
Whisk cider vinegar, rapeseed oil, and mustard to dress the watercress.
Plate watercress, top with sliced steak, mushrooms, and crisp shallots.