Asparagus Risotto

1 pound asparagus, peeled, trimmed and cut into one-inch-long pieces, tips reserved 4 to 6 cups chicken or vegetable stock 2 tablespoons extra virgin olive oil 3 tablespoons butter ½ cup onion, diced 1 1/2 cups Arborio or Carnaroli rice 1/2 cup dry white wine 1 lemon zest and juice Salt to taste 1/2 cup …