Preheat oven to 375 degrees F. and line a baking sheet with foil. The foil should be big enough to fold over and seal the edges.
In a small bowl, whisk together the lemon juice, garlic, and melted butter.
Place fresh spinach on the prepared baking sheet, then put salmon on top of the spinach. Top the salmon with tomato slices and onion rounds. Pour the butter mixture over the top, then season with salt, pepper, oregano, and red pepper flakes.
Fold the sides of the foil over the salmon, making sure it is well sealed, so the sauce does not leak.
Place the baking sheet into the oven and bake until cooked for about 12-16 minutes. The baking time will vary depending on the thickness of the fish. Open the foil and check the salmon every 2 minutes after 12 minutes. If you like a crispy top, open the foil after it is cooked through and broil for 2-3 minutes.
Remove from oven and transfer the salmon and vegetables to your serving plate. Spoon some of the butter sauce left in the foil onto the salmon and garnish with parsley and lemon wedges.