Cook the lentils according to package instructions, then drain, reserving a little cooking water. Return both to the saucepan and mix in a lug each of oil and vinegar. Season to perfection and leave with the lid on to keep warm.
Trim and finely slice the scallions (if using) and put them into a bowl. Squash in the garlic through a garlic press, then stir in the mustard, honey, 1 tablespoon of vinegar, 3 tablespoons of oil and a pinch of salt and pepper. Mix well to make a dressing, and put aside.
Boil the eggs in a saucepan of boiling salted water for 6 minutes, then scoop out with a slotted spoon and run them under cold water to cool. Trim the ends off the beans, then add the beans to the water to blanch for 4 minutes, or until just cooked but not squeaky when you bite into one. Meanwhile, shred the chicken. Drain the beans, and while they’re still hot, toss with a little dressing, the chicken and tarragon leaves (if using).
Tip the warm dressed lentils onto a large platter. Shred the outer leaves of the lettuce, cut the center into quarters and arrange on top of the lentils. Sprinkle over the dressed beans and chicken, drizzle over the remaining dressing, and toss it all together. Peel and quarter the eggs and pop them in and around the salad, then tuck in.