Step 1Pass the ricotta through a food mill or fine mesh strainer into a bowl. Stir in the eggs, low-moisture mozzarella, fresh mozzarella, Parmigiano-Reggiano, nutmeg and salt. Taste and add more salt or nutmeg if necessary.
Step 2Stuff each squash blossom with a spoonful of the ricotta mixture, being careful not to overstuff -- the ricotta should not be forcing its way out of the flower. Figure 1 1/2 to 2 tablespoons per blossom.
Step 3Place the rice flour in a medium bowl and slowly whisk in enough sparkling water to make a batter the thickness of light cream, about three-fourths of a cup. Prepare a large bowl of ice water and set the bowl with the batter in the ice bath to keep it cold.
Step 4Fill a deep, wide pan with canola oil to a depth of at least 2 inches. Heat over high heat to 375 degrees. Dredge the stuffed squash blossoms in the batter, making sure they are completely covered, then drop them into the hot oil, frying only 3 or 4 at a time to avoid overcrowding. Fry until the blossoms are golden brown, 2 to 3 minutes.
Step 5Remove the blossoms to a tray lined with paper towels to drain briefly, then transfer to a platter. Sprinkle with sea salt and a few drops of lemon juice and serve immediately.