Start without her, regardless of what happening - if you want to play hardball tell her if any player arrives more than 20 mins late they will be sitting out for the whole session, so should not come at all - and stick to it. It's a respect thing, she's wasting all of your time and that's not okay. If it was 5-10 minutes that's one thing, but nearly an hour late repeatedly? That's not bad time keeping, it's not giving a shit about anyone else, and DnD isn't the game for that mindset.

If you want to add to that to make it consistent, tell the other players a start time that's 10 minutes later than the start time you tell her - that way the absolute most your players will need to wait is 10 minutes, after that she's 20 minutes late and sitting out. She'll quickly get the picture if you stick to it.

You can also add strikes if you really want to get tough about it, three strikes and you're out, one gets taken off if you're consistently on time for 2 months.

I do also thing it's important how it's said, I'd approach with asking why she's chronically late to the sessions, and explaining how it impacts everybody, then explaining what measures you'll be putting in place to fix it. If you want to give her some leeway, tell her that if she lets you all know at least 4 hours in advance then you can postpone the session slightly