Preheat oven to 350F. Line a large baking sheet with parchment paper and arrange the puff pastry shells on the sheet.

Prepare the filling. Preheat a large frying pan over medium heat and add the 2 tablespoons of butter. Melt the butter, then add the diced onion and chopped asparagus. Sweat the mixture without browning for 8 to 10 minutes until the onions are translucent. Add the brown sugar and vinegar, turn up heat to medium-high and allow the sugar to caramelize.

Meanwhile, combine the cheese, salt and seasonings in a large bowl. When onion mixture is ready, add that to the bowl, too. Using a spatula, mix everything together until a uniform mixture forms.

Fill shells with a heaping amount of filling. Garnish the top with asparagus tips or chopped fresh herbs, such as chives, parsley or dill. Bake in preheated oven for 15 to 20 minutes. Remove and allow the tarts to cool slightly before serving.

NOTE: these tarts are great for freezing. Place the prepared tarts onto a sheet and place into the freezer. When baking from freezer, preheat oven to 375F and bake until piping hot, about 20 to 25 minutes.