I would insist on not sharing. Insisting on sharing is strange to me.
But to be fair, she should be allowed either to choose how well-done the steak is or be allowed to insist on sharing.
It's unjust for her to force her barbaric preferences on you. It's bad enough that she makes you cook it that way for her. But if she won't let you cook your own steak the way you like it, and forces you to eat the one that she made you overcook? That's starting to sound like she loves steak more than she loves you. Which I'm sure isn't true. But it's nevertheless a weird thing to do to your beloved.
