Warm tomato and onion salad with a beef dressing and sourdough croûtons

Name: Warm tomato and onion salad with a beef dressing and sourdough croûtons
Ingredients: 2 red onions; drizzle rapeseed oil; ½ loaf stale sourdough bread; 300g/11oz beef dripping; salt and black pepper; 1kg/2lb 4oz mixed tomatoes (preferably heritage varieties), cut into equally sized chunks; 1 strong English onion, thinly sliced; 2 tbsp lemon thyme leaves; 2 tbsp sea salt; 1 bunch spring onions, chopped, white and green parts kept separate; 3 tbsp cabernet sauvignon vinegar; 1 bunch chives, finely chopped; 2 tbsp toasted onion seeds