Preheat oven to 275 degrees Fahrenheit
In a large skillet, heat 2 tablespoons of olive oil over low-medium heat. Add the sliced onion, bay leaf (optional - use the freshest possible), a pinch of salt. Cook, stirring occasionally until onion is golden brown, about 20-25 minutes. Keep the heat low, and do not rush as you do not want the onion to brown too fast as you want them onion to become soft and translucent.
While the onions are cooking, season the salmon fillets with salt and pepper. Set aside until the entire sauce is cooked (more of those instructions below).
Once onions are cooked, add olives, capers, raisin, and 2 Tbsp water to the skillet and combine to cook. Stir occasionally until the olives are soft (approximately 4 minutes).
Add the rum and cook until the liquid is evaporated and soaked into the other ingredients (approximately 5 minutes).
Remove the skillet and stir in the lemon juice and 1 Tbsp water. Feel free to add a dash or two more of water to make a thinner sauce (only if needed).
Season with salt.
Cut four large sheets of parchment paper.
Once the sauce is complete, place the parchment on a baking sheet (so you do not have to move them once filled), fill each parchment paper with the following ingredients and steps: Spoon one-quarter of the sauce into one side of the parchment; create a well in the sauce the approximately size of the salmon (almost creating a nest for the salmon); place salmon in the nest; drizzle with olive oil; now fold over the side of the parchment paper that is empty - enveloping your salmon. Gather the two edges and fold as though you were dog-earring the pages of a book. Fold at an angle all the way around to the other side. It won't look perfect, but so long as the paper remains closed, the heat will primarily stay inside the parchment while cooking (view my IG Story Highlights - Recipes - to see how it looks when it comes out of the oven - @thesimplyluxuriouslife).
Bake in the oven until salmon is cooked through, about 20-22 minutes (thicker cuts will need more time). Upon removing from the oven, let steam on a cooling rack for 2 minutes.
Unwrap at the table with guests or in the kitchen. Finish with a wedge of lemon is desired and enjoy with a lovely glass of crisp white wine or a rosé.