Simple ratatouille is probably the world's most versatile side dish - it goes with anything. I actually am perfectly happy to have it on its own with crusty bread.
Purists of French cooking, look away now. This is a shortcut, a hack, a timesaver, a classic made easy. It's not that I don't know how a ratatouille should be cooked by the book: each kind of vegetable sauteed separately, layered in turn in a casserole that eventually goes in the oven for the flavours to mingle. You can do the migling on the hob but still going through the stages of onions, peppers, courgettes and so on, probably washing the pan after each vegetable. Tomatoes should be the heritage type, scalded and skinned; you should use fresh herbs and by no means add mushrooms. So I admit that this is a completely unorthodox version – but man, is it quick. And easy.
As it happens ratatouille is very much frowned upon anyway in its homeland. It's no more than a 'vulgar' dish, as I learned from an entry on TripAdvisor commenting on an actually very lovely Provençal restaurant. The reviewers were French - which is fine since it's best to trust the natives on food - and they derided what the chef presented as légumes confit referring to it as ‘une vulgaire ratatouille’ Bof bof.
So there is my justification: since the poor dish is thought to be vulgar anyway, no one should take offence if I cut corners, add mushrooms, ketchup and cook it super quickly all in one pan.
The only chore is dicing the vegetables, the actual cooking takes no more than fifteen minutes or thereabouts. The ratatouille benefits from resting a little once cooked so if it is to be a side dish, you can happily cook the fish or meat while it's standing.
Variations (apart from my mushrooms) might include adding meat of some kind: I'd say French Toulouse sausages will be excellent, or lean smoked bacon. It will turn into a French version of shakshuka if you poach eggs in it. Spoon it cold onto a thick bruschetta and grate cheese over it before sliding it under the grill. But frankly, just a crusty baguette alongside a bowlful of ratatouille is nothing to bof at.