In a medium saucepan, warm the stock over medium-low heat. Cover and keep warm.
In a round-bottom pan or large skillet, heat the oil, two turns of the pan, over medium-high. Add the pancetta, if using, and stir 1 minute. Add the scallion whites and garlic; stir 1 minute. Add the rice and sage; stir 1 minute. Season with salt and pepper. Add the wine; simmer until evaporated, 1 to 2 minutes.
Add about 2 cups stock. Simmer, stirring occasionally, until evaporated. Add another 2 cups stock. Simmer, stirring occasionally, until the stock is almost evaporated.
Add the peas, scallion greens and the remaining stock. Simmer until the rice is tender but still saucy, 4 to 5 minutes more. Stir in 1/2 cup cheese, the herbs and the butter; season. Serve in shallow bowls topped with pea shoots and lemon zest, passing more cheese at the table.