Sauteed Cod with Lentils

For lentils

1 cup dried lentils (preferably French green lentils* often called lentilles du Puy); 7 ounces)
2 tablespoons unsalted butter
1 cup finely chopped onion (1 large)
2 large garlic cloves, chopped
3/4 teaspoon salt
3 tablespoons chopped fresh flat-leaf parsley
1 tablespoon fresh lemon juice
1/4 teaspoon black pepper
1 tablespoon extra-virgin olive oil plus additional (optional) for drizzling

For fish

4 (5- to [...]