Heat the olive oil and butter in a large saucepan over medium heat. Add the leeks and garlic, and cook until softened.
Add the Arborio rice and cook, stirring constantly, until the rice is lightly toasted.
Pour in the white wine and cook until it has been absorbed by the rice.
Add a ladleful of vegetable broth to the rice, stirring constantly until the broth is absorbed. Continue adding broth, one ladleful at a time, until the rice is creamy and cooked through.
Stir in the asparagus pieces and cook until tender.
Remove from heat and stir in the grated Parmesan cheese. Season with salt and pepper to taste.