Can you cast a commander from the command zone during your opponent's turn if it has flash?

Flash (CR 702.8) allows a permanent spell to be cast any time you could cast an instant — meaning on any player's turn, during any phase or step where the stack is legal to add to, as long as priority is held. This applies regardless of where the spell originates, including the command zone.

The command zone is simply an alternative casting zone for your commander (CR 903.13). Casting a commander from the command zone follows all normal timing rules for casting spells; the only special rule is that you may pay an additional commander tax (2 generic mana per previous cast). Flash overrides the default sorcery-speed timing restriction for permanent spells (CR 307.1).

So if your commander has flash, you can cast it from the command zone at instant speed — during an opponent's main phase, their combat phase, or in response to another spell or ability. All normal priority and stack rules still apply.

Concrete example: Your commander is Teferi, Mage of Zhalfir (which has flash). It's your opponent's end step and they pass priority. You hold priority and cast Teferi from the command zone, paying his mana cost (plus any commander tax). This is perfectly legal because flash grants instant-speed casting.

Note: If something removes flash from your commander (e.g., your opponent controls Teferi, Time Raveler), you would be restricted back to sorcery speed and could only cast your commander during your own main phase with an empty stack (CR 307.1).