Can you use flash to cast a creature during your opponent's draw step?

Flash (CR 702.8) allows you to cast a creature spell as though it had flash — meaning you can cast it any time you could cast an instant. Normally, creature spells can only be cast during your own main phase when the stack is empty (CR 307.1), but Flash removes that restriction entirely.

During your opponent's draw step, after they draw their card, they receive priority (CR 117.3b). If they pass priority and you receive it, you may cast a creature with Flash at that point. The draw step is not a main phase, but since Flash grants instant-speed casting, that doesn't matter (CR 702.8a).

The key timing requirement is simply that you must have priority and the rules must not otherwise forbid casting (CR 117.1). There is no rule preventing you from casting Flash creatures during an opponent's draw step specifically.

Concrete example: Your opponent is in their draw step and passes priority to you. You cast Briarskein Familiar (a creature with Flash) in response to nothing. It resolves immediately since the stack is otherwise empty, and the creature enters the battlefield before your opponent's precombat main phase begins.

Note: If a card says "you may cast this only during your turn" or similar, that overrides Flash. But absent such a restriction, the draw step cast is perfectly legal (CR 702.8a, 307.1).