Make tomato sauce:
Cook garlic and red pepper flakes in oil in a 2-quarts heavy saucepan over medium heat, stirring, until garlic is golden, about 30 seconds. Add tomatoes, water, sugar, and 1/2 teaspoon salt and simmer, uncovered, stirring occasionally, until thickened, 25 to 30 minutes.
Prepare squash blossoms:
Stir together ricotta, yolk, mint, 1/3 cup parmesan, and 1/8 teaspoon each of salt and pepper.
Carefully open each blossom and fill with about 2 rounded teaspoon ricotta filling, gently twisting end of blossom to enclose filling. (You may have filling left over.)
Whisk together flour, remaining 1/3 cup parmesan, 1/4 teaspoon salt, and seltzer in a small bowl. Allow batter to sit for about 5 minutes to thicken.
Heat 1/2 inch oil to 375°F in a 10-inch heavy skillet. Meanwhile, dip half of blossoms in batter to thinly coat. Fry coated blossoms, turning once, until golden, 1 to 2 minutes total. Transfer with tongs to paper towels to drain. Coat and fry remaining blossoms. (Return oil to 375°F between batches.) Season with salt. Serve with tomato sauce.
For tomato sauce:
- 1 garlic clove, minced
- 1/4 teaspoon hot red pepper flakes
- 2 tablespoons olive oil
- 1 1/2 pound plum tomatoes, finely chopped
- 1/2 cup water
- 1/2 teaspoon sugar
For squash blossoms:
- 1 cup whole-milk ricotta (preferably fresh)
- 1 large egg yolk
- 1/4 cup finely chopped mint
- 2/3 cup grated Parmigiano-Reggiano, divided
- 12 to 16 large zucchini squash blossoms
- 1/2 cup plus 1 tablespoon all-purpose flour
- 3/4 cup chilled seltzer or club soda
- About 3 cups vegetable oil for frying