Bring a large pot of salted water to a boil. While the water comes to a boil, trim and peel the asparagus. Shell the fava beans and the peas, keeping them separate. Prepare a large bowl of ice water.
When the water boils, add the fava beans. Boil for 1 minute. Use a slotted spoon or strainer to lift them out of the boiling water and into the ice water.
While they cool in the ice water, add the asparagus to the boiling water. Cook until just tender, about 3 minutes. While they cook, lift the fava beans out of the ice water and onto a clean kitchen towel or layers of paper towels to dry.
When the asparagus is cooked, lift it from the boiling water to the ice water.
Do the same with the green peas, cooking them in the boiling water for about 2 minutes—transferring the asparagus out of the ice water first.
Double-shell the fava beans: remove and discard the lighter green shell to reveal the bright green bean underneath.
Finely chop the pancetta. Heat a small frying pan over medium-high heat, add the olive oil and cook the pancetta until its fat has melted off and the pancetta is well browned, about 5 minutes.
While the pancetta cooks, trim and mince the green garlic (or garlic and green onion). Add it to the pancetta and cook, stirring, until the green garlic softens, about 3 minutes. Remove the pan from the heat.
Zest about 1 teaspoon of zest from the lemon and set aside. Cut the lemon in half and squeeze about 2 tablespoons lemon juice into the pan, stirring to release any browned bits that have stuck to the bottom.
Arrange the asparagus spears on a platter and spoon the liquid from the pan over them. Sprinkle generously with salt and pepper. Use your hands to sort of roll them around to coat the asparagus with the “dressing.”
Add the fava beans and peas to the platter. Sprinkle everything with the lemon zest, plus more salt and pepper. Spoon the remaining pancetta and green garlic over the top.