Preheat the oven to 200˚C (390˚F). Half the tomatoes and scoop out the seeds into a bowl. You'll be surprised how much water there actually is in them. Mix the halved and deseeded tomatoes with the thinly sliced onion, olive oil, salt and pepper.
Spread the mix onto a baking tray lined with baking paper. Roast for about 20 minutes until the tomatoes have dried out slightly and the onions are cooked through.
Add the vegan butter or coconut oil to an ovenproof pan of about 20-25cm (8-10inch). I love to use cast iron pans, so it's a natural choice. It gives the tarte-tatin and nice structure, and you do not have to worry about putting it into the oven. Melt the butter, add the crushed garlic to it and fry at medium heat for about 2 minutes. Make sure the heat is not too high, you want the garlic to soften, not burn. Add the sugar and balsamic vinegar and stir. Let the mixture lightly caramelise for another minute.
Pour the roasted onion and tomatoes into the pan and push them down while leaving a little bit of space to the rim of the pan. Roll out the puff pastry sheet on a floured surface if you bought it in a block - I buy mine already rolled out. Cut out a circle slightly larger than the pan. Cover the tomatoes and tuck the corners carefully under. Pierce the puff pastry with a fork and place the pan into the oven. Increase the heat to 200˚C (440˚F) and bake for 20-25 minutes until golden brown.
Take it out of the oven once it is golden. Let the tomato tarte-tatin rest for about 10-15 minutes. Run a knife around the edge of the puff pastry, cover it with a plate and flip it over. Remove the pan and serve immediately warm or as a delightful snack when it cooled down.
If you make this you can also simply double the amount of tomatoes and use half as delicious antipasti, sandwich topping or snack. They keep well for several days in the fridge when placed in an airtight container or mason jar and you can add extra seasoning as well if you want to.