Place the salt and herbs into a medium mixing bowl and incorporate it very well using both hands until it has a consistent color and texture with no clumps. You have just made green salt. Set it aside.
Take ½ of the sliced onions spread them in an even layer on a large sheet pan. Scatter 10 bay leaves and 5 cloves of smashed garlic evenly on top of the onions. Set aside.
Sprinkle ½ of the green salt evenly onto the meat-side of the duck legs (as opposed to the skin side). Place them meat side down on the sheet pan on top of the onions, garlic, and bay leaves. Sprinkle the remaining green salt over the duck legs. Layer with the remaining sliced onions, garlic and bay leaves. Refrigerate uncovered for 2-3 days. Periodically drain or dab up some of the moisture from the sheet pan with a paper towel as the process moves along. (It’s the combination of the salt and the refrigerator dehydration that will give the confit its luscious texture. Otherwise it will just taste braised.)
After 2-3 days remove the duck legs from the sheet pan and discard the marinade. Pre-heat the oven to 225 degrees. Using paper towels wipe the green salt from both sides of the duck legs completely. Do not rinse. Place them snuggly together at the bottom of a 3 quart ovenproof casserole. Cover with 2 pounds of heated duck fat and place in a pre-heated 225 degree oven for 3 ½-4 hours or until the flesh is easily pierced with fork. Check the duck periodically. Keep the mixture at a slow bubble, not a full boil.
Remove the casserole from the oven. Let the mixture cool on the stovetop until the duck is easy to handle. Scoop out the duck legs one at a time and let the fat drain from each leg as much as possible. Place them snuggly in the bottom of a large airtight container. Using a medium mesh strainer, strain enough duck fat into the airtight container to completely cover the duck legs with an inch of fat. Cover tightly and store in your refrigerator for up to 4 months. Freeze any remaining duck fat for future use.