Heat your oven to 220C/Fan 200/Gas 7. Put a baking tray on the top shelf to heat up. Cut the peppers into 1-2cm slices, discarding the seeds and white bits. Halve, peel and slice the onion so they're about the same width. Toss with 1 tbsp oil. Season. Tumble onto the warm tray. Roast for 10-15 mins or till a little charred around the edges.
Drain and rinse the beans. Tip into a pan and pour in enough water to cover. Simmer for 10 mins.
Peel the garlic. Add it to the beans after 5 mins. Drain the beans and garlic, keeping back a little of the water.
Purée in a food processor or blender with the cumin, the leaves from the thyme sprigs, 1 tbsp red wine vinegar, 3 tbsp of the bean water from the pan and a good pinch of salt and pepper. No food processor? Mash everything together with a potato masher.
Whisk the remaining vinegar with 1 tbsp olive oil. Season to taste. Toss the watercress through the dressing.
Spoon the butter bean purée on plates to create a bed for the salad. Drizzle a little oil over. Dust with paprika. Toss the roasted pepper and onions with the watercress and pile on top. Finish with a little more paprika and olive oil.