Preheat the oven to 180C/350F/Gas 4.
Peel the red onions keeping them whole. Trim the root end and then cut them into 5mm/¼in wedges, keeping the root base intact so they don’t fall apart. Drizzle with a little rapeseed oil, place on a roasting tin and roast in the oven for 20 minutes, or until caramelised.
Break the bread into bite-sized chunks and drizzle them with half the beef dripping. Bake them in the oven until toasted and crisp. Season with salt and pepper as soon as they come out of the oven so it takes the flavour on as they cool down.
Place the tomatoes into a large mixing bowl. Add the English onion and thyme leaves. Add the sea salt and mix thoroughly. Leave to infuse and break down for 15 minutes.
After 15 minutes there will be a lot of water in the bowl. Drain the tomato mixture in a colander and then transfer the tomatoes to a baking tray and put into the oven for 1-2 minutes to warm through.
When the tomatoes are warm, place them in a large bowl with the roasted red onions, the white parts of the spring onions and sourdough croûtons.
Warm the remaining beef dripping in a pan with the cabernet sauvignon vinegar (it should be about one part vinegar to three parts dripping). Mix in the chives, onion seeds and the green parts of the spring onions. Season to taste with salt and pepper.
Dress the tomato salad with the beef dripping dressing and serve immediately.