How to Make It
Bring the stock to a simmer in a medium saucepan. Add the asparagus and cook over moderately high heat until barely tender, about 3 minutes. Using a slotted spoon, transfer the asparagus to a plate to cool. Cut the asparagus into 1-inch lengths. Cover the stock and keep at a bare simmer.
Heat 1/4 cup of the olive oil in a large saucepan. Add the onion, leek and thyme and cook over moderate heat, stirring occasionally, until softened, about 6 minutes. Add the rice and cook, stirring, until slightly golden, about 1 minute. Add enough hot stock to cover the rice and simmer, stirring constantly, until almost evaporated. Add the wine and stock to cover and simmer, stirring, until absorbed. Continue adding more stock and stirring constantly until it is absorbed. The risotto is done when it is still a little loose and the rice is tender but slightly firm, about 25 minutes total. Stir in the spinach and then the asparagus. Season with salt and pepper.
Spoon the risotto into bowls and top each serving with a slice of mozzarella. Drizzle with balsamic vinegar and olive oil and serve.