Step 1Sprinkle the salmon evenly with sea salt to taste, about one-fourth teaspoon on each fillet. Set aside for about 30 minutes to bring to room temperature.
Step 2Heat the oven to 225 degrees. (If your oven does not allow that, set it at 200 or 250 degrees.)
Step 3Brush about 2 tablespoons olive oil over a shallow baking dish just large enough to hold the salmon in one layer. Strew the onion slices over the oil. Brush the fillets lightly with olive oil (about 1 teaspoon per fillet) and place them on the onions, skin side down.
Step 4Bake until the salmon is done to your taste. If you like it almost raw, it will be done in about 10 minutes; cooked through, it will take up to 30 minutes (a thin-bladed knife inserted into the thickest part will penetrate easily).
Step 5Remove the salmon from the oven (you can discard the onions). Sprinkle the salmon with the chopped fines herbs and serve hot or cold.