6 cups vegetable stock or low-sodium broth
1/4 cup extra-virgin olive oil
1 small onion, finely chopped
1 pound arborio rice (2 cups)
1 large zucchini (10 ounces), cut into 1/2-inch dice
4 scallions, thinly sliced crosswise
1/2 cup frozen peas, thawed
One 1/2-pound bunch of arugula, large stems discarded, leaves coarsely chopped
Kosher salt and freshly ground pepper
Freshly grated Parmesan cheese, for serving (optional)