In 2 ½ quart pot, combine lentils, ¼ teaspoon salt, 2-ounce chunk of onion, turmeric, fennel seeds, and stock. Cover pot and bring to boil. Reduce heat, and simmer with lid ajar for about 20-25 minute or until lentils are tender. Remove onion and discard. Transfer lentils to medium bowl and set aside.
In 10-inch sauté pan, heat oil over medium flame. Cut remaining 14 ounces of onions into sauté slices and sweat with ¼ teaspoon of remaining salt until translucent. Add fennel and continue to sweat until onions and fennel caramelize (turn brown and soft), stirring often to prevent burning, about 20-25 minutes.
Add lentils to pan with onions and fennel and simmer on low flame until all liquid is absorbed. Season to taste.
Garnish with scallions and fennel fronds.