how do i teach my border collie to not guard the food

Celtic or celti (my border collie) has the habit of guarding the food while eating it and it is only toward my other dogs and idk much about her past since i got her as a rescue and she is 3 going on 4 years old and i keep telling her share the dish since my other dogs seem to share a dish or even take turns perfectly but Celtic has the motive to just sit or lay by the dish and just chow away even though she just ate… and she has meat on her bones (though according to my mom, Celtic is getting a bit “fat” from eating to much. she is a sweet and lovable girl but the guarding of the food is one thing i am not sure about how to stop or to show her that the other dogs are able to eat too
Some background information i do know
1) she has had 2-3 owners/homes before i got her
2) she has had puppies before
3) Celtic is spayed (though i am the one who paid for it)
4) she was probably (though not 100% sure) abused or mistreated before
5) her food (again not 100% sure) was probably taken away from her multiple times
6) she does have a kennel or carrier that she came with.

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