What You Need:
- 3 white onions, thinly sliced
- extra-virgin olive oil
- kosher salt
- Black pepper
- 1 package frozen puff pastry, thawed
- 1 pre-cooked kielbasa, thinly sliced
- grated Swiss cheese
- fresh thyme
- Preheat oven to 350 degrees. In large skillet, sauté onions in olive oil over medium heat until deeply golden and caramelized, about 20 minutes. Season with salt and pepper.
- Unroll two sheets thawed puff pastry. On floured work surface, roll out puff pastry until slightly thinner. Prick both sheets all over with fork, and transfer to parchment-lined baking sheet. Spread tops with onions, kielbasa, and Swiss, leaving half-inch border around all sides. Bake tarts until puffy and golden, and cheese is melty, about 25 minutes.
- Sprinkle with fresh thyme and season with salt and pepper. Slice into squares for serving