Preheat oven to 350 degrees F. Sprinkle salt on cod filets.
Whisk together lemon juice and mustard in a small bowl.
Prepare 4 pieces of parchment paper as done in video.
Put a small handful of onions near the middle of the parchment paper, next to the fold. Place a cod filet on top. Repeat for each filet.
Layer chopped tomatoes, onion, minced garlic, capers, pepper, and rosemary on top of each cod filet.
Spoon on 1/4 of the lemon/mustard mixture and a Tablespoon of olive oil on each filet.
Close packet by folding edges and overlapping as done in the video.
Bake at 350 F for 15 minutes for perfectly flaky fish.
