First, preheat oven at 350 degrees.
You will need a piece of parchment paper large enough so all the ingredients will be able to fit and a baking sheet.
Slice your red onions, lemon, green onions, and cherry tomatoes. Set aside.
Place your slices of red onions on the bottom of the parchment paper.
Place the slices of lemon on top.
Add the green onions.
Add the cut cherry tomatoes.
Place the Mackerel in the center.
Season the Mackerel with salt and pepper.
Add a drizzle of soy sauce.
Squeeze half a lemon on top.
Add the rest of your green onions on top.
Fold over and crimp the parchment paper edges so no steam is let out.
Place in the oven for no longer than 10 minutes at 350 degrees.
Once the Mackerel is done, cool for 5 minutes. Then cut open the parchment paper.