i've never commented on what she eats to her, as i'm so worried she'd fall down the same path i did.
sadly there is no surer way of bringing this about than anxiously monitoring her weight
even if you don't say anything she will notice if you are looking at her to see if she is gaining weight (as you must be, as you were saying you would restrict her food if she gained weight)
she will notice if the thought of her eating (or not eating) makes you anxious
if I were you I would work very hard to find something else to be interested in while you're eating together
don't look at her plate, don't add up what she is eating, distract your mind, find something fun to talk about
bake with her if you think that would be a fun thing to do, not to allay your anxieties re her food intake
your eating disorder and your fear of that coming back is your cross to bear, part of your burden in life
your dd should not have to engage with that fear
She is 15, not 5, so shouldn't have her life that closely monitored anyway. Why is she not allowed to eat in her room? Do you have a super-precious carpet? If I were you I would relax rules at a time like this: teens need a certain amount of unsupervised time and just at the moment she can't have it like she ought to.