How do I support my child's choice to stop eating meat without cooking two complete dinners?

My twelve-year-old moved the chicken to the side of her plate and said, "I don't want to eat animals anymore. I'm not asking you to stop."

I answered that I wasn't making two dinners. She pointed at the rice and vegetables and said she could eat those. That solved tonight's argument in her mind, but not my question about what her meals would include over time.

She still wants eggs and dairy, so she's asking for a vegetarian diet rather than a vegan one. [NHS guidance emphasizes varied foods for children and attention to nutrients including iron and vitamin B12](https://www.nhs.uk/live-well/eat-well/how-to-eat-a-balanced-diet/the-vegetarian-diet/). I don't want to assume that simply removing the meat creates a suitable whole diet, or use that concern to dismiss her choice.

She offered to help plan and prepare food. I can welcome that without making her fully responsible for feeding herself. How would you organize shared meals and the extra learning involved, while keeping her ethical decision separate from whether the rest of the family agrees with it?