1ouncefinely grated Parmiagiano about 1/2 cup plus additional for serving
1/2cuphazelnutstoasted and coarsely chopped
Cut top third of each asparagus stalk diagonally into 1/2-inch-thick slices, reserving tips and slices together, then coarsely chop remainder. Bring water (5 1/2 cups) and 1/2 teaspoon salt to a boil in a 3- to 4-quart saucepan, then add chopped asparagus and cook, uncovered, until very tender, 6 to 7 minutes. Transfer with a slotted spoon to a food processor (not a blender, which would require adding liquid).
Add reserved asparagus tips and slices to boiling water and cook, uncovered, until crisp-tender, 2 to 3 minutes. Transfer with slotted spoon to a sieve, reserving cooking liquid in pan, and rinse under cold water to stop cooking. Drain well and reserve in another bowl.
Measure cooking liquid and, if necessary, add enough water to bring total to 4 cups, then reserve.
Cook onion with pepper and 1/4 teaspoon salt in oil in a 4- to 5-quart heavy pot over moderate heat, stirring occasionally, until softened, 5 to 7 minutes. Add barley and cook, stirring, 1 minute.
Add wine and boil, stirring, until liquid is absorbed, about 1 minute.
Add 4 cups reserved asparagus-cooking liquid and bring to a boil, covered, then reduce heat and simmer, covered, until barley is tender (it should be chewy) and mixture is thickened to a stewlike consistency, 35 to 40 minutes.
Meanwhile, mince garlic and mash to a paste with remaining 1/4 teaspoon salt using side of a large heavy knife, then add to asparagus in the food processor along with zest and purée until smooth.
When barley is cooked, stir in asparagus purée, asparagus-tip mixture, and enough additional water to thin to desired consistency and cook over moderate heat, stirring, until hot, about 1 minute. Stir in cheese, then season with salt and pepper. Serve with hazelnuts and additional cheese on the side.
Barley can be cooked in about half the time in a 6- to 8-quart pressure cooker. Follow recipe, cooking onion in pressure cooker, uncovered, then adding barley and wine as directed above.
After adding asparagus-cooking liquid, seal pressure cooker with lid and cook at high pressure, according to manufacturer's instructions, 18 minutes. Put pressure cooker in sink (do not remove lid) and run cold water over lid until pressure goes down completely.
Remove lid and continue with recipe, using pressure cooker (without lid) as a pot. Asparagus can be cut and cooked, chopped stalks puréed, and cooking water reserved 1 day ahead, then chilled in separate airtight containers.