Paris where to eat before opera bastille

FWIW. lots of people bring sandwiches and eat during the intermissions and such food is also vended at the site. What we do in Paris is have a cafe lunch that day -- more like a dinner and then just get a snack at the apartment or carry small sandwiches and goodies to eat at the opera during intermissions. Any of the cafes in the area will do for a quick bite before -- there are several ringing the Bastille square.

The majority of traditional restaurants do not serve dinner before 7 p.m.
Where you can eat a real meal at any time are the "brasseries"
In Bastille near the Opera there are several.
For example La Grande Brasserie Bastille:

https://www.grandebrasserie.fr/en/

Otherwise if you just want a salad or a sandwich, you will be spoiled for choice

Just in Paris (about 3 weeks ago) and had to picnic it before the Opera since most of the near by venues did not open in time. By the way, the opera (and opera house) were spectacular.

I have not been yet Brasserie Rosie on Rue Faubourg de Saint Antoine (about a 5-minute walk from the opera house) offers continuous service and opens at noon.

We stayed for a month right near Bastille Opera - just got home last week. If you don't need a full restaurant, there is a cafe we enjoyed a few times right near the opera, Le Bastille. You can have a nice meal and people watch, then stroll 2 minutes over to the Opera.

consider going to happy hour and getting a planche i.e. charcuterie/cheese board with bread -- they are substantial and tasty and will serve as a light dinner before the opera -- have the more balanced meal at lunch

Brasserie Bofinger...it's just across la place.