Make the Choux Pastry (Cream Puff Shells):

Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.

In a medium saucepan, combine water, milk, butter, salt, and sugar. Bring to a boil over medium heat.

Once boiling, remove from heat and quickly stir in the flour all at once. Return to heat and cook, stirring constantly, for 2–3 minutes until the dough forms a ball and a thin film coats the bottom of the pan.

Transfer dough to a large mixing bowl or stand mixer with a paddle attachment. Let cool for 5 minutes.

Beat in eggs one at a time, ensuring each egg is fully incorporated before adding the next. The final dough should be thick, smooth, and shiny.

Spoon or pipe 1.5-inch mounds onto the prepared baking sheet, spaced about 2 inches apart.

Bake at 425°F (220°C) for 10 minutes, then reduce temperature to 375°F (190°C) and bake for another 20–25 minutes or until golden brown and hollow-sounding when tapped. Do not open the oven door early!

Remove and cool completely on a wire rack. Slice tops off with a serrated knife when ready to fill.

2. Prepare the French Onion Meatloaf Filling:

Heat olive oil and butter in a skillet over medium-low heat. Add onions, salt, and sugar, and cook slowly for 25–30 minutes, stirring occasionally, until caramelized and golden brown. Add Worcestershire sauce and cook 2 more minutes. Set aside to cool slightly.

In a large bowl, mix ground beef, breadcrumbs, milk, egg, garlic powder, black pepper, and caramelized onions. Mix until just combined—do not overmix.

Transfer to a baking dish and bake at 375°F (190°C) for 25–30 minutes or until internal temperature reaches 160°F (71°C). Let rest 5 minutes.

Crumble or finely chop the meatloaf. Stir in shredded Gruyère while warm.

3. Assemble the Cream Puffs:

Spoon or pipe warm French onion meatloaf mixture into the bottom half of each cream puff.

Replace tops and garnish with a sprinkle of parsley or extra cheese if desired.

Serve warm, or reheat briefly in the oven before serving.