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
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
Equipment: a deep-fat thermometer
Make tomato sauce:
Sauté garlic and red pepper flakes in oil in a 2-quart 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.
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.