In a 12-ounce or larger microwave-safe mug, whisk together the flour, sugar, baking powder, salt, and poppy seeds with a fork until evenly distributed.
Add the milk, oil, vanilla extract, lemon zest, and lemon juice to the dry ingredients. Stir gently with the fork just until the flour disappears and no dry pockets remain, being careful not to overmix.
Tap the mug gently on the counter twice to release any large air bubbles.
Microwave on high power for 70 to 90 seconds, checking at 70 seconds. The cake is done when the top looks just set and springs back lightly when touched. It will continue to cook slightly after removing from the microwave.
Let the cake cool in the mug for 30 seconds before eating directly from the mug or carefully inverting onto a small plate.
Top with powdered sugar, whipped cream, or a simple lemon glaze if desired. Serve immediately while warm.