1️⃣ Prep & Preheat

Preheat oven to 400°F (200°C).

Unfold puff pastry and cut into 4 equal squares (or leave whole if using a larger pan).

Place on a parchment-lined baking sheet. Use a fork to lightly prick each square (if not bubbling over filling). Set aside.

💡 Pro Tip: Keep pastry cold until ready to bake — helps it rise higher!

2️⃣ Caramelize the Onions

In a skillet over medium-low heat, melt butter with olive oil.

Add onions and sugar. Cook 8–10 minutes, stirring occasionally, until soft and golden brown.

Stir in garlic, cook 30 seconds.

Deglaze with white wine or broth, scraping up browned bits. Simmer 1 minute.

3️⃣ Build the Filling

Add chicken, thyme, salt, and pepper. Stir to combine and warm through.

Remove from heat. Stir in Swiss and Parmesan cheeses and Worcestershire (if using). Mix until gooey and well blended.

4️⃣ Assemble the Bubble Ups

Spoon the chicken mixture evenly over each puff pastry square (or pile in center if making one large tart).

Fold edges slightly inward (optional), leaving the top exposed.

5️⃣ Egg Wash & Bake

Brush exposed pastry edges with beaten egg for golden shine.

Bake 15–20 minutes, until pastry is puffed, deeply golden, and crisp.

The filling will bubble around the edges — hence the name! 🎉

6️⃣ Rest & Serve

Let cool 2–3 minutes (filling will be molten hot!).