Secret ingredient: Lemon - Part 1

Here in the US there is a TV show called the Iron Chef where a few chefs are competing with a limited amount of time to prepare a few dishes and just as they are about to start they are given a "secret ingredient" by the judges to be included in all the different dishes.
The last dinner we hosted I decided to have my own "secret ingredient" and to integrate it in all my dishes.

This secret ingredient was Lemon.

And the first dish was a Velvety Asparagus Soup with a tinge of lemon
For 6
Ingredients
2 bunches of fresh asparagus (I used green ones)
1 medium size shallot, peeled and chopped
6 pints of chicken broth (or 6 pints of water and 1 chicken broth cube)
Zest of 1/4 of a lemon (you don't want to overpower the soup with lemon)
Juice of 1/2 a lemon
2 TBSP Olive oil
1.5 TBSP Butter
2 TBSP flour
Salt and pepper to taste

Method
Wash and peel the asparagus, the easiest way is to use a potato peeler with the swivel head and rest the asparagus flat on a board.
Cut the heads of and reserve them.
Cut the stems in approx. 2"/5cm long sticks.
In a cast iron pot (or other) on high heat fry the shallot in a bit of olive oil, then add the asparagus sticks, after 5 mn pour the chicken broth/water + cube, add salt and pepper to taste and grate 1/4 of a lemon zest and add the juice of half a lemon and let simmer covered on gentle heat for 30mn or so.
While it is simmering steam the asparagus heads in a separate pot, do not overcook, they should be a bit crunchy (5 to 8mn should be enough).
Prepare a "roux" in a separate pot: melt 1.5 TBSP butter, sprinkle 2 TBSP flour and whisk, then use a ladle to pour some of the simmering broth on the butter/flour mix and whisk adding more broth as needed till you obtain a smooth liquid custard looking mix.
Blend the simmering asparagus soup with a hand held blender then pour the custard looking mix in the blended asparagus and let cook for another 10mn.
When ready to serve place asparagus heads cut in 2 length ways in the soup and serve immediately.

Note
I like using the cast iron pot for this as it cooks using less energy since it retains the heat better, it looks nice as a serving dish and keeps the soup hot and it is convenient as you do not need to transfer the soup to another container.