Place eggs in a saucepan of boiling water and simmer for 6 minutes. Remove eggs from saucepan and transfer to a large bowl of iced water. Crack shells gently all over to prevent any further cooking and a dark edge forming around yolk.
To shell, lightly tap eggs on a board and roll briskly under your hand; or tap the shell lightly and peel a strip around centre of egg so two halves slip off. Cut eggs in half lengthways.
Combine sour cream, mayonnaise and 2 tablespoons of herbs in a bowl. Season to taste. Add 1 finely chopped egg and toss gently.
Combine green onion, asparagus, celery and baby spinach in a serving bowl. Toss salad gently and top with remaining egg halves, cut sides up. Sprinkle with capers and poppy seeds and serve with herb dressing.