Put potatoes in a large pan of salted water, bring to boil and cook until tender but still holding their shape and not falling appart. You might need to check on them few times by piercing them with a knife to check whether it will go in easily. Once they're done drain them and set aside.
Whilst your potatoes are cooking prepare asparagus by snapping off the woody ends. You can either steam or briefly boil your asparagus until just tender. Cut each cooked asparagus spear into three pieces, sprinkling them with lemon juice and setting aside. Keep asparagus tips separately from the stalks.
Heat up tablespoon of oil in a pan and fry chopped onion and pancetta (if using) until onion is softened and translucent and bacon is starting to get crispy.
Prepare your dressing by putting all the ingredients in a small jar and shaking it vigorously to mix. Season to taste with salt and pepper and shake some more.
Add lentils to the pan with onion and bacon and mix well. Keep the pan warm for a couple fo minutes to allow for the lentils to warm through.
Add cooked potatoes and follow with the dressing.
Add cooked asparagus stalks and mix them in.
Arrange the asparagus tips on top of the salad, sprinkle generously with chopped dill or parsely and extra squeeze of lemon juice.
Serve immediately, still warm on a bed of mixed salad leaves with some delicious crusty bread.