Preheat your oven to 400°F and lightly grease a 12-cup muffin tin. Roll out the thawed puff pastry and cut into 4-inch squares. Gently press each square into the muffin cups, allowing edges to overlap slightly – this creates those beautiful rustic edges that'll turn golden and crispy. The pastry should feel cool and pliable, not sticky. If it's too warm, pop it back in the fridge for 10 minutes.

Heat a large skillet over medium-high heat and add the ground beef. As it sizzles and browns, break it apart with a wooden spoon, listening for that satisfying searing sound. Once the beef is about halfway cooked (around 4 minutes), add the diced onions. You'll smell the onions becoming fragrant and see them turning translucent – this usually takes another 3-4 minutes. Stir in the garlic during the last minute, just until aromatic.

Drain any excess fat, then stir in ketchup, mustard, and Worcestershire sauce. The mixture should look glossy and smell just like your favorite burger joint. Season generously with salt and pepper, then remove from heat. This step is crucial because it builds those classic cheeseburger flavors that'll make these tarts irresistible.

Divide the beef mixture evenly among the pastry-lined muffin cups, filling each about three-quarters full. Top generously with shredded cheese – don't be shy here! The cheese will melt and bubble beautifully, creating that golden top we're after. Brush the exposed pastry edges with beaten egg; this ensures they'll turn gorgeously golden brown.

Bake for 18-20 minutes, until the pastry is puffed and golden brown and the cheese is melted and slightly bubbly around the edges. You'll hear gentle bubbling sounds and smell that incredible combination of buttery pastry and savory beef. The pastry should feel firm when lightly tapped.

Let the tarts cool in the pan for 5 minutes – this prevents them from falling apart when you remove them. Carefully lift each tart out and sprinkle with fresh chives for color and a mild onion flavor that complements the beef beautifully.