Heat a medium saucepan on medium-low heat. Add butter and finely chopped onion. Cook, stirring occasionally, until the onion softens, about 5 to 7 minutes.
Add farro, white wine vinegar, and salt, and stir well for 1-2 minutes to remove any coating on the bottom of the saucepan. Add water and bring to a boil. Reduce heat to low, cover, and cook for 20 minutes.
After 20 minutes, begin tasting the risotto. You want the grains to be tender, but still a little chewy. It could take as long as 45 minutes to reach this stage. Add more water if needed. It is okay if there is a little liquid left in the end. In fact I prefer that.
Five minutes before the farro is cooked to your liking, add finely chopped asparagus. When you prepare the asparagus, separate the stalks from the tender spears. Add the chopped stems first and cook for 2-3 minutes, then add the spears, and cook until asparagus is just tender.
Turn off the heat and pulse half the dish with an immersion blender to create a smoother texture, and make it easier to digest the farro. Stir in the chopped parsley and freshly ground black pepper. Taste, and adjust for salt.
Dish risotto into individual bowls. Add optional garnishes, such as shaved Parmigiano Reggiano and hazelnuts. Sprinkle on more black pepper and a little fresh lemon if you like.
Notes
Asparagus farro risotto is also wonderful topped with feta cheese. Or replace the parsley with fresh cilantro, basil, oregano, and/or tarragon.