Reserve a few rocket leaves for serving then put the remaining rocket, spinach and watercress in a salad bowl. Add the cherry tomatoes and set aside.
To make the cannellini bean purée, put the beans, garlic, lemon juice, thyme, oil and 2 tablespoons of water in a food processor or blender. Season with salt and pepper, to taste, then process to a smooth, soft purée. Add a little more water, if necessary. Transfer to a saucepan and heat gently for about 5 minutes, stirring frequently, until piping hot. Alternatively, put the purée in a microwaveable bowl, cover and cook on high for about 4 minutes, stirring halfway through, until piping hot. Let stand for 1 minute before serving.
Meanwhile, to cook the salmon fillets, heat the oil in a non-stick frying pan. Add the garlic and fry gently for 1 minute. Add the salmon and cook for 5–8 minutes, turning once halfway through.
Drizzle the salad with a little balsamic vinegar, season to taste with salt and pepper and toss well. Put a spoonful of cannellini bean purée on each of 4 warmed serving plates. Put the reserved rocket leaves on top, followed by the salmon. Drizzle with a little extra virgin olive oil. Serve immediately with the salad.
Variation: The bean purée would be delicious with most grilled foods. Try chicken, turkey, large prawns or tuna. Use other beans or a mixture of two or three types and keep the purée slightly chunky, if you prefer.