Dear My Wonderful Dream Agent:
On the date of 00/00/09, I submitted a query to you for my novel, I Can't Believe You Didn't Like It. You rejected this story, but also told me that should I write a contemporary YA novel, you would like me to submit it to you.
You don't want to start off with the specifics about having been rejected.
Start with the hook and query and then put the "last time I queried you you said send me one of this kind of novel" in the last paragraph.
So:
Dear Agent:
On ARTICHOKE ISLAND, peas snap in terror as celery stalks by midnight. Jane Doe was lured by the promise of vegging out in a tropical paradise, but soon found that the Hitler-cloning space aliens had transformed this Eden into a virtual Hell.
Jane's tae kwon do skills soon come in handy, as she chops and slices her way through the enemy to rescue Chad Hunk, the fiance she had believed killed at sea. But the little green men in brown shirts have other plans for Jane and Chad...like breeding a half-human half-veggie master race.
When I queried you in August, 2007, you said you'd be very interested in reading a thriller with a strong vegetarian angle. I think that ARTICHOKE ISLAND, which is complete at 90,000 words, more than meets that definition. The full manuscript is available on request.
Sincerely,
Doctor Vegetable Moreau