Cook the onions & prunes

In a large fry pan over medium heat, warm the olive oil. Add the sliced onions and the salt. Cook the onions down, stirring occasionally, until they just start to turn a golden brown, about 25-30 minutes. The goal is to *almost* caramelize the onions in the pan since they will continue to brown in the oven.

Add the chopped prunes during the last few minutes of cooking the onions. Then transfer the mix to a large bowl and allow to cool to room temperature.

Shape & bake the tarts

Preheat oven to 400ºF and prepare the egg wash by whisking together 1 egg with small splash of water.

Unroll the thawed puff pastry sheet. It should be pliable and easy to unroll but still slightly cold to the touch.  If the pastry is too warm, it can become sticky and hard to work with.

Cut each sheet of puff pastry into a 3x3 grid of 9 squares. Puff pastry sheets are often folded into 3rds so cutting along the seams is a natural place to cut.

Transfer the squares to a lined baking sheet, evenly spaced apart. Gently brush each square with the egg wash.

Use a knife to score a 1/2 inch border around around the edges, without cutting all the way through the dough. Then use a fork to lightly prick holes in the middle of the squares. This will create a border that frames the filling and rises as the tarts bake.

Spread a generous teaspoon of mustard in the center square of each pastry square. Top with a heaping spoonfuls of the prune and caramelized onion mix. Then sprinkle about 1 heaping tablespoon of grated Fontina over the tops.

Bake for 13-15 minutes, until the frames puff up, containing the filling. The edges should be a warm golden color and the cheese should be browning and bubbling.

Finish & serve the tarts

Drizzle with honey and top with small sprigs of thyme. Serve immediately while warm and puffy!