Prepare the spinach and herb pesto: Put the spinach and herbs in a food processor or a blender. Process, adding a little stock to loosen if needed, until well combined. The pesto should resemble a very thick soup. Set aside.
Make the risotto: Bring the stock to a boil; reduce to a simmer. In a medium, heavy-gauge saucepan over medium-high heat, melt 2 Tbs. of the butter. Stir in the rice, toasting just until it starts to sizzle and pop, about 1 minute. It should not color. Add the onion, stirring constantly, and cook until translucent, 1 to 2 minutes. Stir in the wine.
When almost all the liquid has disappeared, after about 1 min., add just enough hot stock to cover the rice. Lower the heat to maintain a vigorous simmer; stir occasionally. When the stock is almost gone, again add enough stock to cover the rice, along with a pinch of salt. Check the risotto every 3 or 4 minutes, giving an occasional stir to make sure it isn't sticking to the bottom of the pan, adding just enough stock to cover the rice when the liquid has almost disappeared.
Continue this way until the rice is just al dente, about 20 minutes total cooking time. Bite into a grain; you should see a white pin-dot in the center. Take the risotto off the heat. Add the remaining 1 Tbs. butter. Stir in half of the herb pesto (freeze the rest for another time) and the cheese. Stir vigorously for a few seconds. The risotto should be moist and creamy, not runny. Add more stock to loosen the risotto if you like, and more salt to taste, if needed. Serve immediately.