This is definitely a can of worms but if you search using ‘nutrition’, there’s a bunch of threads on the subject. Personally, fuel with the next workout in mind. Take a look at The Endurance Diet by Matt Fitzgerald; I follow it as closely as I can and I can tell you that it has made a difference for me.
For lunch, I eat the same thing everyday at work because it works for me- Cuban Beans with Brown Rice - (from the book above) or, sometimes, change it up with red beans and rice. Point is, I prep ahead for the week for lunch and dinner on Sunday. My breakfast (after training) is oats (porridge) with flax seed meal.
Check out this thread about meal prep - it’s really got a ton of great information
Good luck!