Preheat the oven at 350 degrees F. Peel the red onions, keeping them whole. Trim the root ends and then cut into 1/4-inch wedges, keeping the root ends intact so they don¿t fall apart. Drizzle them with a little rapeseed oil and roast on a rimmed baking sheet until caramelized, about 20 minutes.
Break the bread up into bite-size chunks and drizzle with 5 ounces of the beef dripping. Bake on a rimmed baking sheet until toasted and crispy. Immediately season the croutons with salt and pepper and let cool.
Cut the tomatoes into same-size chunks and place them in a large mixing bowl. Thinly slice the strong onion and add it, along with the thyme leaves, to the tomato bowl. 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. Turn the tomato mixture into a colander to drain. Transfer the tomato mixture to a rimmed baking sheet and roast just until warmed through, 1 to 2 minutes.
In a large bowl, combine the tomato mixture, roasted red onions and white parts of the spring onions. Mix in the crusty bread croutons.
Warm the remaining 5 ounces of beef dripping in a small saucepan. Add the vinegar, taste and season. Add the onion seeds, chives and green parts of the spring onions. Dress the salad with the warm beef drippings mixture and serve immediately.