Start by seasoning the salmon fillets with 2 tablespoons of oil, minced garlic, minced ginger, salt, turmeric, and ground cumin. Allow the salmon to marinate for at least 15 minutes to absorb the flavors.
In a large pot, heat 1 tablespoon of oil over medium heat. Add the finely chopped onion and sauté until translucent. Stir in the minced garlic, ginger, and fresh curry leaves, cooking until fragrant.
Add the dried split red lentils to the pot along with the broth. Season with 2 teaspoons of salt, 1 teaspoon of ground cumin, 1 teaspoon of turmeric powder, ground fennel, ground coriander, and garam masala. Bring to a boil, then reduce heat and let it simmer until the lentils are tender, about 20 minutes.
While the lentils are cooking, heat another tablespoon of oil in a frying pan over medium heat. Add the salmon fillets, skin-side down. Cook for 4-5 minutes on each side or until the salmon is cooked through and flakes easily with a fork.
Once the lentils are cooked, taste and adjust seasoning if necessary. Gently place the cooked salmon on top of the daal. Serve hot, garnished with half-moon onions sautéed with cumin and mustard seeds.