Eeek 7-8 hours out of a fridge is quite a challenge!
I’d second a previous suggestion of cold cous cous paired with something like ratatouille with Parmesan shavings and added chick peas for protein.
Those upside down kilner jar salads are good for keeping things very fresh. You put the dressing in first and then layer up the other ingredients. You can even put croutons on top if you want. Then when you want to eat them, you turn them upside down and the dressing is properly distributed. Maybe add a small tin of tuna at the last minute?
Noodles with a peanut butter dressing and Asian veg would be good in a jar.
A variation on the theme is home made Aikii-type noodles salad that has lots of flavour from a spicy dressings with dried and fresh mushrooms.
Maybe too salad-ish though?
How about one of those pressed sandwiches? Hollow out a small loaf and put aside insides for breadcrumbs. Brush out the two hollowed-out shells of loaf with light coating of olive oil. Then layer up roasted veg inside, like peppers and strips of courgette and mushrooms, rocket, basil, garlic, etc. Add slices of mozzarella. Then put bread halves together, press down, wrap tightly in foil and put in fridge with a heavy weight on top overnight.
Maybe veggie samosas?
Edited: sorry just read your update about mushrooms!
Pesto would be good in a pressed roasted Med veg sandwich too.