I'm having family over for lasagna and minestrone. My niece responded to the invitation with a request that the onions in the lasagna be finely minced! Since the onions are diced, then cooked not once, but three times in the course of becoming part of the lasagna, I'm sure they will be indistinguishable in the final product. I was stunned by her request, but assured her that the sauce would be acceptable. This is a 23-yr. old person who has a degree in "restaurant/hotel mgmt./hospitality industry." I wish I could find a gentle, respectful way of telling her that this sort of correspondence is terribly rude. BTW, this same niece failed to respond to a Thanksgiving meal invitation until 6 p.m. the night before. Then, she arrived late with an additional (surprise) person in tow. The rest of us were already finished eating by that point.
If you had a history of adding whole onions to your lasagna then maybe, maybe I could get behind your niece's unsolicited cooking request. Otherwise, I agree that her appeal for finely minced anything is rude. Your question was how to tell your niece that this behavior is rude, presumably because you love her and want her to do well out in the world where others might respond with less kindness.
I think you should find a quiet moment to ask your niece in person about her request. Ask her why she made it. My guess is her answer will be something along the lines of "because that's the way I like lasagna" or "I don't like onions so am hoping I won't taste them if they are minced into microscopic bits." I hope that hearing herself say this aloud might bring about some self-awareness. If not, remind her that when she's invited to dinner (to your home or anyone's), she is a guest and that it is both unreasonable and ungracious to send specific cooking instructions in advance. Express how surprised you were at her request to drive home the point that her behavior was way off the beaten etiquette track.