Dean Richards shares his recipe for French Onion Tart Cups. Printable recipe below. 

Ingredients:  

• 1 ½ lb. onions (about 4 small), preferably yellow, halved, thinly sliced
• 3 Tbsp. unsalted butter
• ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt, plus more
• 1 garlic clove, finely chopped
• 2 tsp. sherry vinegar or red wine vinegar
• 2 tsp. Worcestershire sauce
• 1 tsp. freshly ground pepper
• 4 oz. Gruyère, coarsely grated, divided
• 1 sheet frozen puff pastry (one 14-oz. package or half of a 17.3-oz. package), thawed
• 1 Tbsp. Dijon mustard
• Chopped chives (for serving)

Directions:

1. Pre-heat oven to 400 degrees.
2. Combine 1½ lb. onions (about 4 small), preferably yellow, halved, thinly sliced, 3 Tbsp. unsalted butter, and salt in a large Dutch oven or other heavy pot over medium, cover, and cook, stirring occasionally, until onions are softened and translucent, 9–11 minutes.
3. Uncover and continue to cook, adding splashes of water (up to ½ cup) and stirring and scraping bottom of pot often, until deep golden brown, 15–20 minutes.
4. Remove pot from heat and stir 1 garlic clove, finely chopped, 2 tsp. sherry vinegar or red wine vinegar, 2 tsp. Worcestershire sauce, 1 tsp. freshly ground pepper, and 2 oz. Gruyère, coarsely grated, into onion mixture. Taste and season with more salt if needed; set aside.
5. Preheat oven to 400°. On a lightly floured surface, unfold puff pastry. Roll each sheet into a 12-in. square; cut each into 9 squares. Place squares in ungreased muffin cups, pressing gently onto bottoms and up sides, allowing corners to point up.
6. Par-bake the puffed pastry 5-7 minutes.    Remove from oven.   Let them cool.  
7. With a pastry brush put a thin layer of Dijon mustard and a tablespoon of shredded cheese at the bottom of the cup.   
8. Add onion mixture.
9. Top with more shredded cheese.
10. Bake tart cups until golden brown and cheese is melted, 7-10 minutes. Let cool 5 minutes.
11. Top with chopped chives.