Lay out 4 sheets of parchment paper. The size should be approximately 16" x 12"
Rinse and remove the spinach leaves from the stems. Dry before placing them on the parchment paper.
In a saucepan, boil the potatoes until par-boiled, half done. About 7 - 10 minutes. Drain, let cool and thick slice. Watch them carefully, you don't want them cooked all the way, they'll become mushy when steamed if cooked too long.
Heat a fry pan over medium high heat. Add oil and when oil is hot add In a sauce pan, add sliced mushrooms and rings of red onion. Cook until the onion starts to turn opaque and the mushrooms have given up their moisture. About 3 minutes.
Place the fresh spinach leaves on each parchment paper sheets. Top with a fish filet, seasoned to your liking. I used Old Bay for this version.
Top each filet with a slice of the lemon, and a sprig of rosemary. Sprinkle on the onions and mushrooms, evenly on each pouch meal. Add the potatoes and the tomatoes. Top each with a pat of butter. Salt and pepper to taste.
Add a small splash of white wine over each meal.
Fold the parchment paper over the food and start crimping the edges over each other to seal.
When ready to cook, preheat oven to 400 degrees. Place pouch meals on a baking sheet. Cook 10 - 12 minutes.