In a medium bowl, combine ricotta, mozzarella, Parmigiano-Reggiano, egg, salt, and nutmeg.

Stir well to combine.

Adjust seasoning with salt and nutmeg to taste.

Trim the squash blossom stems, leaving about 1 inch.

Make a lengthwise incision on each blossom to open it gently.

Remove the stamen from each blossom.

Open each blossom and spoon about 1 tablespoon of the cheese mixture inside.

Close the blossom by twisting the petals.

Prepare an ice water bath.

In a separate bowl, whisk rice flour and sparkling water until smooth.

Maintain a thin pancake batter consistency, adjusting water as needed.

Place the batter bowl in the ice bath to keep it cold.

Heat grapeseed oil in a medium saucepan to 350°F (175°C).

Line a baking sheet with paper towels.

Working in batches, dip each squash blossom in the batter, ensuring it's fully coated.

Carefully drop the battered blossoms into the hot oil stem-first.

Fry for 3-4 minutes, turning occasionally, until golden brown.

Remove blossoms from the oil with a slotted spoon and drain on paper towels.

Squeeze lemon juice over the fried blossoms and sprinkle with sea salt.

Serve immediately.

Clean cooked bits from the oil with a slotted spoon between batches.

Maintain the oil level and temperature at 350°F for subsequent frying.