Leek and asparagus soup too watery

Or, you could add some cooked farro into the blender when you reduce the soup.

Simmering/reducing would help, but it will sacrifice the color and freshness of the asparagus. My first choice would be to add more asparagus; blanch/shock them and puree with some of the soup you already have.
If you're looking to just thicken in, there are options:
Rice; ladle some of the soup into a pot and cook some rice in it (ratios and timing not important since it will be pureed anyway).
Bread (preferably old bread); just heat the soup a little and add a couple handfuls of torn bread in. Kill the heat and let it soften for about 10 minutes, then puree. *Toasting the bread first will impart nice flavor.
A roux; cooked separately in a pan and tempered with a couple ladles of hot soup, whisking constantly, then added to the soup.
Liaison (mixture of egg yolks and cream), tempered with some hot soup and stirred in. This will only thicken it slightly, but the flavor and mouthfeel it adds is great.

Hey! You could just stir in some mashed potatoes. This way you won't have to cook the whole thing for even longer meaning you'd lose the colour of the veg. Depending on your mash recipe it could add more depth of flavour. For an even easier cheat you could use instant mash granules but might be a bit wallpaper paste-like.

Just reduce it! Simmer it on medium-low, uncovered, until it gets to your desired thickness. The flavor will also be stronger at that time, so don't add any seasoning until it is as thick as you want.