Heat the olive oil in a large skillet over medium-low heat and add the sliced onions. Cook for 15-20 minutes or until the onions have turned brown and caramelized; set aside to cool.
Once the puff pastry has thawed, roll out one sheet onto a floured surface and cut into 6 squares, doing the same with the other sheet to create 12 pastry squares total.
To assemble the hand pies, spread a thin layer of honey mustard on 6 of the pastry squares, followed by a slice of turkey, a small handful of spinach, then grated cheddar. Cover the square with another piece of puff pastry and crimp the edges with a fork. Repeat until each hand pie is assembled.
Brush the tops with heavy cream and sprinkle with any remaining cheese. Bake at 425 for 20 minutes or until puffed and golden.