Heat the olive oil and butter in a medium saucepan over medium heat. Add the leeks and fennel and saute for 5 to 7 minutes, until tender. Add the rice and stir for a minute to coat with the vegetables, oil, and butter. Add the white wine and simmer over low heat, stirring constantly, until most of the wine has been absorbed. Add the chicken stock, 2 ladles at a time, stirring almost constantly and waiting for the stock to be absorbed before adding more. This process should take 25 to 30 minutes.

Meanwhile, cut the asparagus diagonally into 1 ½-inch lengths and discard the tough ends. Blanch in boiling salted water for 4 to 5 minutes, until al dente. Drain and cool immediately in ice water. (If using fresh peas, blanch them in boiling water for a few minutes until the starchiness is gone.)

Whisk the lemon juice and the mascarpone together in a small bowl. When the risotto is done, turn off the heat and stir in the mascarpone mixture plus the Parmesan cheese and chives. Set aside, off the heat, for a few minutes, sprinkle with salt and pepper, and serve hot with a sprinkling of chives and more Parmesan cheese.