Turn the oven to 180 celsius or 350 Fahrenheit. Meanwhile, add the sliced onions to a pot containing 4cm/2" of cold water and cook over a low heat for 20-30 minutes stirring frequently until the onions are soft and they begin to caramelise. Keep adding more water if the water evaporates. You may have to do this three or four times. This helps to makes the onions lovely and soft and breaks down the natural sugars in the onion.
Put the puff pastry on a flat surface like an oven proof baking sheet. Use a pastry cutter or saucer and cut out four circles, each measuring 12cm/5" in diameter.
Turn over the edges of each and pinch them to help them retain their shape. Prick the bases with a fork and using a pastry brush, brush them with some plant-based milk such as almond or brown rice.
Line the tart with baking parchment and fill it with ceramic baking beans or dried pulses. This helps to stop the base becoming soggy and is known as 'baking blind'.
Put the tarts into a hot oven for five to ten minutes.
Remove from the oven and discard the baking parchment and ceramic baking beans or dried pulses.
Divide the cranberry sauce equally between the four pastry circles. Top with the caramelised onions and sprinkle the freshly topped rosemary over the top of the four tarts.
Bake for 15-20 minutes until they're well risen, then serve with your choice of vegetables and potatoes.
Brush the edge with some plant based milk then bake for 15-20 minutes until well risen.