Bring two pans of water to the boil. Simmer the asparagus for 1 minute, then drain and plunge into a bowl of ice-cold water to stop it cooking.
Wilt the spinach with a splash of water in a large pan, then leave to drain any excess liquid off over a sieve.
Heat the oil in a wide griddle pan and cook the asparagus spears for 3-4 minutes, until there are dark golden griddle marks all over. Keep warm.
Add the vinegar to the other pan of boiling water. Crack an egg into a small bowl or ramekin. Using a slotted spoon, swirl the water so that a whirlpool forms, then carefully pour the egg into the water. Cook for 3 minutes, then remove with a slotted spoon and drain on kitchen paper. Repeat with the remaining 3 eggs.
Divide the asparagus between 4 plates, then top with the spinach, poached egg, smoked salmon and hollandaise sauce. Garnish with the dill.