Preheat the oven to 200°C, gas mark 6.
Place the onions, oil and 1tbsp vinegar on a baking tray, toss together and roast for 15 minutes. For the croutons, tear the baguette into bite sized pieces and toss with the pesto until evenly coated. Scatter over the onions and bake for a further 10 minutes until golden.
Meanwhile, mix the spinach, tomatoes and remaining vinegar together, toss in the onion and croutons and serve immediately.
Try using sun dried tomato paste instead of pesto or try adding some quartered boiled eggs.
This recipe is from the British Leafy Salads Association, which looks to inspire people to make more of salads. Learn more here: www.makemoreofsalad.com