Heat your oven to 180°C/Fan 160°C/Gas 4. Line a baking tray with baking paper. Brush it with 1 tsp oil. Lay the hake on the tray, skin side down. Bake for 15-20 mins till opaque and cooked through. The fish should flake easily when you press it with a fork.
While the hake bakes, rinse the quinoa under cold water for a few mins. Tip into a pan with 150ml boiling water. Cover. Simmer for 12-15 mins till the water has been absorbed and the quinoa is tender. Stir now and then.
Measure out 1 tsp of the sesame seeds. Add them to a dry frying pan. Toast over a low heat for 2-3 mins till the seeds are golden and smell nutty. Tip into a large bowl.
Halve the cucumber. Run a teaspoon down the middle of each half to scoop out the seeds. Slice the cucumber into little crescents. Add to the sesame seeds and toss to combine.
Halve the chilli. Finely chop it and add to the cucumber. Pick any thick stalks out of the watercress, so you just have leaves and tender shoots. Set the watercress aside.
Drain the cooked quinoa into a sieve and cool it under cold running water. Tip the quinoa into the cucumber and stir well to combine.
Measure out 1 tbsp of the brown rice vinegar. Whisk it with 2 tsp oil and some seasoning to make a dressing. Add it to the cucumber and quinoa salad. Toss to coat everything in the dressing.
Lift the fish off the baking tray with a spatula – the skin may stick to the paper, but you can leave it behind. Serve the fish with the cucumber and quinoa salad and the watercress.