Thaw corn at room temperature. Finely chop green onion.
In a large bowl, whisk together egg, half-and-half, cornstarch, baking powder, and salt.
Stir in corn, mozzarella, parmesan, and green onion until combined. If batter looks loose or pools at the bottom of the bowl, mix in an extra tablespoon of cornstarch.
Add 1½ tsp oil to each cup of a 6-cup muffin tin and swirl to coat. Preheat oven to 420°F (215°C) and heat the tin until oil is hot, a few minutes.
Divide batter evenly between the hot cups. Bake at 420°F (215°C) for 25-30 minutes, until tops are golden and edges pull slightly from the tin.
Cool briefly and serve warm with a sour cream or yogurt-based dipping sauce.