Warm the stock: Place the stock in a saucepan over medium-low heat. Keep it warm but not simmering as you prepare the risotto.
Saute the asparagus: In a large skillet over medium-high heat, heat 2 tablespoons of olive oil. Add the diced onion, asparagus, garlic, salt, and pepper. Saute for 3 minutes, stirring occasionally. Stir in 2 tablespoons of lemon juice and cook for another 2-3 minutes. The asparagus should turn a bright, vibrant green. Transfer to a plate and set aside.
Cook the rice: In the same skillet, add the remaining tablespoon of olive oil. Stir in the rice and toast for about 30 seconds, until fragrant. Add 3/4 cup of the warm stock to the rice and stir. Reduce the heat to medium and let the rice absorb the liquid.
Build the risotto: Continue adding stock, 3/4 cup at a time, stirring frequently and allowing the rice to absorb the liquid before adding more. This process will take approximately 25 minutes. When finished, the rice should be creamy and tender but slightly firm to the bite.
Finish and serve: Remove the risotto from the heat. Stir in Parmesan cheese, the reserved asparagus mixture, fresh dill, and the lemon zest. Taste and adjust seasoning as needed. Serve hot, garnished with additional fresh dill and a sprinkle of lemon zest.