Nonna's fresh tomato sauce for pasta

INGREDIENTS

• salt
• 18 ripe tomatoes
• 1 red onion, peeled and chopped
• olive oil
• 2 carrots, peeled and roughly chopped
• 2 garlic cloves, peeled and crushed
• 3 basil leaves
• 1 stalk celery, roughly chopped
• 1 white onion, peeled and chopped
• pinches bicarb of soda