Step 1

In a medium saucepan, combine the broth, water, leek, thyme sprigs and ½ teaspoon kosher salt. Bring to a boil over medium-high, then reduce to low and keep at a bare simmer.

Step 2

In a Dutch oven over medium heat, melt 2 tablespoons of the butter. Add the shallot and cook, stirring frequently, until softened, 1 to 2 minutes. Add the rice and stir until the edges begin to turn translucent, about 3 minutes. Add ½ cup of the prosecco and cook, stirring frequently, until the wine is absorbed, about 1 minute. Add about 1 cup of the simmering broth and leeks to the rice and cook, stirring occasionally, until the liquid is almost completely absorbed, adjusting the heat as necessary to maintain a gentle bubble. Continue adding the broth and leeks about 1 cup at a time, stirring occasionally and adjusting the heat as necessary, until the rice is tender, 20 to 25 minutes total. Discard the thyme sprigs.

Step 3

Increase the heat to medium-high, add the remaining ½ cup of prosecco and remaining 1 tablespoon of butter and stir vigorously until the butter is melted. Off the heat, stir in the fontina, Parmesan, 1 teaspoon salt and minced thyme until the cheeses are melted. Divide the risotto among 4 shallow bowls and make a well in the center of each serving. At the table, pour about 1 tablespoon of prosecco into each well. Stir to incorporate. Season to taste with salt and grated Parmesan.