For the filling:
Heat the EVOO in a Dutch oven over medium-high heat. Add the meat, brown and crumble. Remove the meat and drain off the fat. Melt the butter in the same pot and add the zest, oregano, garlic, onions and some salt and pepper. Cook to tender, 12 to 15 minutes. Then add the fresh herbs, stir. Add the flour, stir a minute or so and then whisk in the stock. Cook to thicken a little. Add the meat back in along with the lemon juice and spinach. Combine. Turn off the heat, cool and store for a make-ahead meal. Reheat over medium heat until hot. Stir in the feta crumbles just before serving.
For the puff pastry topper:
Preheat the oven to 425 degrees F. Flour a work surface and lightly roll out the puff pastry. Cut the size and shape of 1 large casserole or up to 6 individual bowls or large ramekins, using sharp paring knife. Place the dough on a nonstick baking sheet and wash with egg. Bake until evenly puffed and deep golden brown in color.
Serve the hot filling in a casserole dish or individual bowls topped with the pastry.