これらのお肉はレア焼きですがよろしいでしょうか?って英語でなんて言うの？

"Rare" meat is meat with a mostly red center.
"Medium-rare" meat is meat with a partially red center.

"Cooked rare" and "served rare" are common expressions.

In restaurants where customers can choose how their meat is cooked, waiters commonly ask, "How would you like your meat cooked?" or "How do you like your meat?"

Seared means put on intense heat and lightly cooked or raw.

"We (only) serve/cook this meat rare. Is this okay with you?"

✰ I think it could be good to use the words "we" and "you" to make this interaction more personal and friendly.
✰ "Serve" and "cook" would have the same effect in this case.
✰ "Okay" and "alright" and "fine" have the same meaning.
✰ Use "only" if you only serve the meat cooked rare. If you offer other kinds of cooked meat such as "medium rare" or "well done", you can follow up with the question "How would you like it (the meat) cooked?"

Example:
GUEST: I'd like to order this item.
SERVER: We serve this meat rare. Is this fine with you?
GUEST: Actually, I don't want it rare.
SERVER: How would you like it cooked?

I hope this helps! Good luck! ✰✰

Both these phrases clearly communicate how the meat will be cooked and both phrases make it clear you are not offering a choice of how the meat will be served.
The meat will be bloody is another way of indicating that it will not be well cooked.

"This dish is served with rare meat" means that this dish comes with meat that is cooked rare.

"Are you okay with that?/Is that okay for you? " are both ways of asking the customer if that is okay.

"This meat in this dish is served rare." tells them how the meat is cooked.

The meat in this dish is served rare. Are you sure you want to order this? - This is clearly indicating that the meat is served rare and you are making sure that they want to order this and will be happy to eat it.

This meat is only served rare. Would you like a small piece to taste before you order it? - This is clearly indicating that the meat is served rare. You are also offering a small piece of the rare meat for them to taste to see if they would like it before they order this meal.

1. Are you OK with...? = This question form is checking that something is alright.
2. Are you happy if...? = This question form does not ask if the customer is literally happy, but is checking that the customer accepts this option.
   A "Are you happy with travelling by train every day?"
   B "Yes, I've got used to it now."

This meat is served rare!
Would you like to try it?
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Rare: A rare piece of meat will have a bright red center, and its outside will be brownish-gray. The center will be warm, but not hot.
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Medium-rare: The center of a medium-rare piece of meat will be slightly warmer and reddish-pink instead of red.
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A medium piece of meat will have a large band of pink through the middle, but will be primarily grayish-brown throughout.
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Medium-well: A medium-well piece of meat will just have a slight hint of pink in the middle. Otherwise, it will be primarily cooked through and will feel quite firm.
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Well-done: A well-done steak should show no hint of pink whatsoever. Its center will be grayish-brown throughout and the outside will be nicely charred.

'Is that okay for you / Are you okay with'' - are both ways of asking the customer if they are happy with something. If it is alright with them.

Are you happy with rare meat? That is how it is typically served. - ''Typically served'' means that it is usually served this way.