Heat the oven to 160°C/320°F. Slice half the lemon and remove the seeds.
Put the trout fillets in a baking dish and season well with salt and pepper.
Put the butter in a small saucepan and heat over a medium-high heat. Cook for a few minutes until the butter starts to brown. Add the olive oil, lemon slices and half the onion slices. Season again and continue to cook until the onion and lemon are starting to crisp. Add the capers.
Pour the butter mixture over the fish. Put in the oven and bake until just cooked through, about 10-12 minutes.
Sprinkle over the dill and nigella seeds. Squeeze over the other half of the lemon and scatter over the sliced onion. Serve.
Nutrition information is an estimate and will vary depending on the ingredients used.