Preheat the oven to 375° F.
Make 7.5×12 inch rectangle from folding the parchment paper in half.
Slice the red onion in slices and place them on the right half of each square leaving space between the onions and the edge of the paper.
On top of the onion place each fish fillet and season with one teaspoon of Greek seasoning spice mix and a few drops of olive oil.
Over each piece of fish, place half of the lemon slices. Outside of the fillets, place the tomatoes and olives with two fresh oregano over the top.
Fold the paper over and fold/crimp the edges to close and seal the packets well.
Transfer them to a baking dish with a baking sheet at the bottom and bake for about 8 to 12 minutes.
Let it cool then serve with the sealed packets on individual plates, without opening them.