Heat oven to 400° F.
Heat 2 tablespoons of the oil in a large saucepan over medium heat. Add the onion and cook for 7 minutes or until soft. Add the garlic, ginger, and cumin and cook, stirring, for 1 minute. Add the lentils and broth and bring to a boil. Reduce heat and simmer, stirring occasionally, until the lentils are tender, about 20 minutes. Stir in the lemon juice, 1/4 teaspoon of the pepper, and 3/4 teaspoon of the salt. Remove from heat.
Meanwhile, in a small bowl, combine the coriander (if using), thyme, and the remaining salt and pepper. Place the fish in a baking dish, drizzle with the remaining oil, and sprinkle the tops with the spice mixture. Bake until the fish is the same color throughout and flakes easily, about 10 minutes. Divide the lentils among individual plates and serve with the fish.
Tip: If sea bass isn't available, you can substitute another firm fish, such as grouper, cod, or halibut.