Heat your oven to 200°C/Fan 180°C/Gas 4. Peel the onion. Slice it into wedges. Pop in a large bowl.
Rinse, then trim the very top of the fennel, keeping any soft fronds for garnishing. Halve the fennel lengthways. Slice it into around six wedges. Add to the onion.
Halve the butternut squash lengthways. Scoop the seeds out of one half. Peel it. Chop into bite-size chunks. Rinse and halve the pepper. Scoop out the white bits and seeds. Slice it. Add to the bowl.
Peel and grate or crush the garlic. Grate or pare the zest from the lemon. Add to the veg with 2 tsp olive oil. Season with salt and pepper. Gently toss all the veg to mix them together.
Spread the veg out in a large ovenproof dish or roasting tin. Rinse the oregano. Pat dry. Scatter half the oregano over the veg. Place in the oven. Roast for 30 mins till the veg are starting to feel tender and char a little.
Pop the haddock fillets on top of the veg. Put the dish back in the oven. Roast for 10-15 mins till the haddock is white and cooked through.
Take the fish and veg out of the tin. Squeeze over the juice from the lemon. Top with any fennel fronds you have and the leaves from the remaining oregano sprigs. Serve straight away.