Heat your oven to 200°C/Fan 180°C/Gas 6. Peel and roughly chop the onion. Halve the pepper, scoop out the seeds and white pith, and roughly chop it. Peel and grate or crush the garlic. Pop them all in a roomy ovenproof dish.
Tip in the tin of cherry tomatoes and stir in 1 tbsp red wine vinegar (save the rest for later), 1 tsp caster sugar, 1/2 tbsp olive oil and a pinch of salt and pepper.
Slide the dish into the oven and roast for 25 mins, stirring halfway through. This is your peperonata.
After 25 mins, take the peperonata out of the oven and nestle the salmon fillets, skin-side down, in the dish. Return to the oven and bake for 12-15 mins till the salmon is pink and cooked through – the flesh will be opaque and will flake easily when pressed with a fork.
While the salmon roasts, tip the remaining red wine vinegar into a bowl and add 1 tsp olive oil and a pinch of salt and pepper. Whisk together to make a dressing, then toss the salad leaves through the dressing.
Serve the salmon and peperonata on warm plates with handfuls of the salad on the side.