Bring 3 3/4 cups water, the vinegar, coriander seeds, lemon zest, bay leaves, 2 tablespoons salt, and the sugar to a boil in a medium saucepan. Add onions, and return to a boil. Reduce heat, and simmer 4 minutes.
Divide asparagus between two 1-quart jars. Add onions, and divide liquid between jars. Let cool to room temperature, about 1 hour. Cover, and refrigerate at least 1 day and up to 1 month.
These pickled vegetables go great with cheese toasts. Simply cut a baguette into slices and sprinkle with grated extra-sharp cheddar cheese. Top with a few pieces of thinly sliced prosciutto, if desired, and a little more cheese, and then broil until cheese is melted. Cut asparagus and onions into bite-size pieces, and top the cheese toasts with the pickles.