Make the salmon marinade: combine 2 tablespoons oil, 2 minced garlic cloves, 1/2 tablespoon minced ginger, 1/2 teaspoon salt, 1/2 teaspoon turmeric, and 1/2 teaspoon ground cumin in a shallow dish; add the salmon fillets and turn to coat. Set aside while you prepare the daal, or refrigerate up to 12 hours.
Heat 1 tablespoon oil in a heavy-based saucepan over medium-high heat. Add the finely chopped medium onion and fry until softened, about 4–6 minutes.
Reduce heat to low and add 6 minced garlic cloves, 1 tablespoon minced ginger, and 8 fresh curry leaves. Cook, stirring, for about 30 seconds until fragrant.
Add 1 cup dried split red lentils, 3½ cups broth, 3/4 teaspoon salt, 1/2 teaspoon ground cumin, 1/2 teaspoon turmeric, 1/4 teaspoon ground fennel, and 1/4 teaspoon ground coriander. Bring to a simmer without a lid, then reduce heat, cover, and cook gently.
Cook the lentils until very soft and mostly broken down, stirring occasionally; this will take about 35–45 minutes total—stir gently every few minutes once thick and simmering to prevent sticking.
Stir 1/4 teaspoon garam masala into the daal and check seasoning; keep the lid on the pan while you cook the salmon.
Heat a large heavy-based skillet over medium heat. Place the marinated salmon fillets presentation side down and sear for 3 minutes, then turn and cook 3 more minutes, or until cooked through to your liking. Remove the salmon and place on top of the daal, cover to keep warm.
In the same skillet, add 2 tablespoons oil and the sliced small onion. Cook over medium-low heat until softened, then increase heat so the onions sizzle and begin to color.
Remove the pan from heat, add 1 teaspoon black mustard seeds, 1 teaspoon cumin seeds, and 6–10 fresh curry leaves; let them sizzle for about 20 seconds.
Pour the sizzling oil, onions, and spices (tadka) over the salmon-topped daal and serve immediately.