Asparagus Risotto

6 cups vegetable stock, preferably homemade1/4 cup olive oil1 small onion, finely chopped1 cup Arborio or Carnaroli rice1/2 cup dry white wine1 bunch asparagus, trimmed, stalks cut into 2-inch lengths1 cup thawed frozen peas1 teaspoon grated lemon zest, plus more for garnish2 tablespoons fresh lemon juice1 cup chopped fresh flat-leaf parsley leaves1/2 cup finely grated Parmigiano-Reggiano, plus more for servingCoarse salt and freshly ground pepperBring stock to a simmer in a medium saucepan.Heat 2 tablespoons oil over medium heat in another saucepan. Cook onion, stirring frequently, until soft, 6 to 7 minutes. Add rice, cook, stirring, until edges are translucent, 2 to 3 minutes. Add wine; cook, stirring, just until evaporated.Add 1/2 cup hot stock; cook, stirring, until almost absorbed. Continue adding 1/2 cup stock in this manner until liquid is creamy and rice is al dente, about 20 minutes total (you may not need to add all the stock). Add asparagus with the last addition of stock, and the peas about 1 minute before risotto is done.Remove from heat; stir in lemon zest and juice, parsley, cheese, and remaining 2 tablespoons oil. Season with salt and pepper. Serve immediately with additional cheese and lemon zest.