In a small mixing bowl, whisk together the flour, sugar, and baking powder until evenly combined.
Add the lemon zest, lemon juice, vegetable oil, and egg to the dry ingredients. Whisk until the batter is smooth and slightly frothy, with no lumps remaining.
Pour the batter into a microwave-safe mug, filling it about three-quarters full to allow for rising.
Microwave on high for about 1 minute and 30 seconds, or until the cake has risen and is set around the edges. You'll hear a gentle bubbling sound as it cooks.
Carefully remove the mug from the microwave, and check the cake’s firmness with a toothpick or fork. If it comes out clean, it’s ready; if not, microwave for an additional 10-15 seconds.
Let the mug cake cool for a minute or two, then enjoy it straight from the mug or turn it out onto a plate. The cake should be soft, fluffy, and fragrant with lemon.