Gently open each blossom and check inside for insects (common with farmers market blossoms).
Remove the pistil or stamen (the stiff center piece) by pinching at the base and pulling; if left in, it adds bitterness.
Do not wash blossoms with water because they wilt instantly. Wipe gently with a damp paper towel only if needed and set prepped blossoms on a clean kitchen towel.
Combine ricotta, mozzarella, lemon zest, basil, salt, and pepper in a bowl. Stir gently without mashing the mozzarella cubes. Taste and adjust salt.
Spoon about 1 tablespoon filling into each blossom through the top opening. Do not overfill; the blossom should close easily without spilling. Twist the petal tips together to seal.
Whisk flour, cold water, and salt in a shallow bowl until smooth, about the consistency of thin pancake batter. Add water 1 teaspoon at a time if too thick. Let the batter rest 5 minutes while the oil heats.
Pour neutral oil to ½-inch depth in a medium heavy skillet over medium-high heat. Heat to 350°F (test: a drop of batter sizzles and floats within 2 seconds). Do not let the oil smoke.
Fry in batches of 3 to 4. Dip each stuffed blossom fully in batter, let excess drip off, and lay gently in the hot oil. Fry 1 to 2 minutes per side until deeply golden and crisp. Transfer to a paper-towel-lined plate. Adjust heat between batches to hold around 350°F.
Sprinkle with flaky sea salt and serve immediately with lemon wedges. These are best eaten within 10 minutes of frying.
Notes
Check inside each blossom for insects before stuffing. Common with farmers market blossoms and completely harmless.
Do not overfill the blossoms. One tablespoon of filling per blossom is the maximum; overstuffing causes them to burst open in the oil.
Oil at 350F is the real target. Too cool and the batter absorbs oil and turns greasy; too hot and the outside burns before the cheese melts. Test with a drop of batter: it should sizzle and float within 2 seconds.
Do not rinse blossoms with water. They wilt instantly. Wipe gently with a damp paper towel if needed.
These do not store, reheat, or travel. Serve within 10 minutes of frying.