Looking at a day trip from London.
Any good restaurants near the train station?
Why would you spend big bucks for a round trip from London on the Eurostar, only to have lunch at a restaurant near Gare du Nord and go back to London?
A little learning is a dangerous thing ;
Drink deep, or taste not the Pierian spring :
Alexander Pope
Because I can
I’ve eaten in the area and it was just OK but if you’re going to see some sights or just walk around there are more pleasant places for a meal.
When are you going?
If the weather is pleasant I love one of the terrace restaurants in the Tuileries. Or any number of restaurants a short walk or Metro ride away.
How about Terminus du Nord, a classic French brasserie across from the station. You didn’t state your price range.
Correction merci JHK: Terminus Nord.
JHK has posted below and always has very good recommendations for Paris restaurants; I would definitely follow any recommendations from JHK.
I was going to recommend Terminus du Nord as well.
Yes, I also liked Terminus du Nord, and here's an additional suggestion, also just across from the station.
https://25hours-hotels.com/paris/terminus-nord/
I stayed there and enjoyed the restaurant very much. It's a bit quieter than the brasserie vibe, and it has an eclectic menu.
If you are willing to walk about 12 minutes, I suggest Les Arlots on Rue du Faubourg Poissonnière and Brasserie Belanger, also on Rue du Faubourg Poissonnière. If you want to only have to cross the street, I agree with the suggestion of Terminus Nord.