Put the spinach in a colander then pour over a kettle of just-boiled water to wilt. Cool, then squeeze out as much water as possible and chop (get it as dry as you can otherwise the rarebit will be soggy).
Put the cheddar, mustard, spring onions, worcestershire sauce and egg yolk in a bowl with some seasoning. Add the spinach and mash everything together.
Poach the remaining 2 eggs in a small pan of simmering water.
Heat the grill to high. Toast the bread on both sides then divide the rarebit mixture between the slices, spreading to cover.
Grill until golden and bubbling, then top with an egg, lots of black pepper and serve with a little dressed rocket.