Tear or chop lettuce into small pieces. Divide among serving plates. Sprinkle evenly with spring onions.
To make vinaigrette, whisk 2 tablespoons of the oil, 2 tablespoons of the vinegar, mustard and horseradish until combined. Add salt and pepper to taste. Drizzle over salad ingredients.
Hall-fill a large saucepan with water. Add remaining 3 tablespoons vinegar; bring to a boil. Heat remaining 2 tablespoons oil in a frying pan. Add bacon; fry until crisp. Keep warm.
One at a time, break eggs onto a plate and slide them into the boiling water. Reduce heat to a simmer. Poach eggs 4 to 5 minutes, or until cooked as desired. Remove with a slotted spoon and drain. Trim ragged edges. (Alternatively, hard–boil the eggs; peel and slice when cooled.) Place 1 egg on each serving of salad. Top with bacon. Serve immediately.