Step 1Heat the oven to 400 degrees. In a blender, grind the garlic, green onions, cilantro and parsley to a thick paste; if necessary, start adding oil.
Step 2With the machine running, gradually add the oil through the feed tube. Season with cumin, salt and paprika and pulse to mix. Add the lemon juice and pulse again. The mixture should be a fairly coarse, full-flavored puree. Adjust the seasonings with salt and lemon juice.
Step 3Remove the side fin and bones if necessary. Scatter the onion slices in a single layer in a large earthenware baking dish. Generously coat both sides of each fillet with the herb paste and place on top of the onions, skin-side up. Cover with aluminum foil and bake 20 minutes.
Step 4Remove the foil and continue baking until the skin begins to turn golden, 15 to 20 minutes. Serve each fillet on its bed of onions.