Prepare the kale: First remove the ribs, then roll the leaves and slice in 2 directions until you have pieces the size of dimes or quarters. Put the shredded kale in a bowl with lemon juice and ½ teaspoon salt and massage gently for about 2–5 minutes. Set aside.

Heat butter in a pan on medium heat. Add onions, ½ teaspoon of salt, and thyme sprigs. Cook for 2–3 minutes, then reduce the heat to medium-low and allow the onions to slowly caramelize for at least 20 minutes. Then, remove the thyme sprigs, add balsamic vinegar and cook for another 5 minutes, stirring frequently and adding water 1 tablespoon at a time, if necessary.

Preheat the oven to 400°F.

When onions are done caramelizing, add the kale to the pan and continue to cook on medium-low heat for 5–10 more minutes. Add water as necessary, but make sure most of the liquid has evaporated when you are done cooking.

Create 8 small tarts by cutting the pastry into squares or 2 large tarts with no cuts, then place the squares on 2 baking pans lined with parchment paper. Edge the pastry by scoring the dough ½ inch around the outside of the squares. Do not cut all the way through the pastry. Using a fork, prick the dough inside the square border. Par-bake the pastry squares for 5 minutes then remove from the oven to fill with toppings.

Evenly divide the kale mixture onto the squares then top with feta and Parmesan cheeses. Bake for 15 minutes or more, until the crust is golden brown.