How do I stop her counter surfing?

[Panicmode1]

This is a new thing - my 2 year old retriever has been good as gold about being left in the kitchen up to now. We have an open plan kitchen diner, so I can't exclude her from the food. Recently, she has taken to jumping up when our backs are turned and stealing food - e.g., half a loaf of bread last week, and just now, I'd left an open Lurpak spreadable on the counter whilst I attended to my 5 year old in the other room, came back, and she'd jumped up, taken it off the counter and eaten half of it.

She is walked for at least 90 minutes a day, we are around at home and the four children play with her a lot, so I don't think that it's boredom - more that she's learnt that she can do it.

HOW do I stop her doing this?! Any ideas? Obviously, at the moment, I have to now put everything away and can't leave her for a second, but it would be nice to get back to the situation we were in before when I could trust her not to steal food.

I have a rescue that I home in January. Within a few hours it was clear he has issues with food, snatching from the kids, off the plates/table/worktop, in the bin.

He made mealtimes a nightmare and would go berserk if locked out. So I have a mat in the kitchen, I taught him the word 'mat' and insist he stays on it if I'm cooking/preparing food or we are eating.

Initially this meant getting up numerous times during a meal, to put him back. And i mean numerous times..Persistence paid off though.

He takes himself to his mat, or I just have to point at it. He doesn't bother is at mealtimes, he just watches from his mat.
I make sure no food is left within reach in the kitchen, if No-one is in there he thinks it's fair game.