How to Make It
In a medium saucepan, bring 5 cups of the vegetable stock to a simmer. In another medium saucepan, heat 2 tablespoons of the olive oil. Add the onion to the oil and cook over moderate heat, stirring occasionally, until translucent, about 3 minutes. Add the rice and cook, stirring, for 1 minute. Add 1 cup of the hot stock and cook, stirring, until the stock is absorbed. Continue adding the hot stock, 1 cup at a time and stirring occasionally until it is completely absorbed before adding more. Continue cooking until the rice is al dente and the liquid is thickened, about 20 minutes total.
Meanwhile, in a large skillet, heat the remaining 2 tablespoons of olive oil. Add the zucchini and scallions and cook over moderately high heat until just tender, about 2 minutes. Add the peas and arugula and cook until the arugula is wilted, about 1 minute. Add the remaining 1 cup of vegetable stock, season with salt and pepper and bring to a simmer. Remove from the heat.
Stir the vegetables and their liquid into the risotto and cook until the rice is creamy, about 1 minute. Season the risotto with salt and pepper. Spoon the risotto into bowls and serve with Parmesan cheese.