Preheat the oven to 350F/175C.
Add the fish filets together with salt, pepper, orange zest and juice (or lemon) to a mixing bowl and let them marinate until the ratatouille is ready.
Heat up a skillet with some olive oil. Add the onion and stir until translucent. Then add the garlic and all the veggies except for the tomatoes. Sprinkle oregano on the veggies and stir for a few more minutes.
Once most of the veggies have softened, add the diced tomatoes and balsamic vinegar. Simmer for 5-6 minutes until the sauce has thickened a bit and the carrots and celery have become soft as well.
Transfer the ratatouille (veggie sauce) to a bowl and set it aside. Add the fish filets to the skillet and add the ratatouille (veggie sauce) on top. Simmer the fish in veggie sauce for 8-10 minutes on low.
Meanwhile make the herb crumble by mixing all herb crumble ingredients.
Sprinkle the herb crumble on top of the ratatouille (that covers the fish filets) and transfer the whole skillet to the preheated oven for 5 minutes.
Serve with mashed potatoes, (brown) rice or quinoa.