Preheat oven to 350°F. Line a cookie sheet with parchment paper or foil for easy cleanup.

Heat olive oil in a 10-inch skillet over medium heat. Add the diced onion and cook, lowering the heat to medium-low, for about 10 minutes, stirring often.

Add the garlic, salt, pepper, and thyme and cook an additional 5 minutes over medium-low heat, stirring often.

Add the Worcestershire sauce and beef stock and stir to combine. Use a wooden spoon or spatula to scrape up the brown bits on the bottom of the pan. Cook until liquid is cooked off and onions are golden and soft, another 3-5 minutes. Remove from heat.

Place phyllo shells on prepared cookie sheet. Fill each with approximately 1 teaspoon of onion mixture. Top with shredded cheese.

Bake 6-9 minutes, or until cheese is melted and tart shells are heated through. Serve warm.

Store leftovers in refrigerator; these are best the same day they’re made but can be reheated in the oven or air fryer.