Heat oven to 220C/200C fan/gas 7. Mix the yogurt, mustard and honey together. Add the lemon zest, then add the juice and some seasoning to taste. Squeeze any remaining lemon juice over the watercress.
Tear the bread into rough chunks and put them on a large roasting tray with the asparagus. Toss both in the rapeseed oil and seasoning, and roast for 10 mins until the asparagus is tender and croutons are golden.
Meanwhile, cook the eggs in a pan of boiling water for 6 mins, then add the frozen peas and cook for 1 min more. Drain and rinse both under cold water until cool. Peel the eggs, then cut into quarters.
To assemble, mix the asparagus and peas through the watercress, then toss through the creamy dressing. Nestle in the eggs and croutons, and serve.