In a medium saucepan, bring the stock to a boil, reduce heat to low, and keep at a bare simmer.
Heat the olive oil in a heavy 4-quart saucepan over medium heat; add leeks, onions, and garlic, and saute, stirring frequently with a wooden spoon, until soft but not browned, about 6 minutes. Add rice, and continue stirring until the edges of the rice are translucent, about 3 minutes. Add wine, and cook, stirring constantly, until nearly all the wine is absorbed, about 30 seconds.
Raise the heat to medium high. Add salt and pepper to taste, and about 1/2 cup of the simmering stock; cook, stirring constantly, until nearly all the stock is absorbed. Continue adding stock, about 1/2 cup at a time; adjust seasonings. Cook, stirring constantly, allowing each addition to be nearly absorbed before adding the next, until the rice is creamy but slightly firm in the center, about 25 minutes.
Meanwhile, fry the squash blossoms. Heat olive oil in a 12-inch nonstick skillet over medium heat until very hot, but not smoking. Prepare batter, by combining egg yolk, milk, flour, and 2 tablespoons cold water with a whisk. Dip each blossom in the batter and gently transfer to the hot oil. It will be necessary to do this in small batches. Fry until crispy and light brown, about 1 to 2 minutes per side. Roughly chop six of the cooked squash blossoms, and gently combine with the cooked rice. Divide the risotto among four plates, and garnish with the remaining fried whole squash blossoms. Serve immediately.