Combine broth and lentils in a small saucepan. Bring to a boil, then reduce heat to maintain a low simmer. Cover and cook until the lentils are tender, 15 to 20 minutes. Remove from heat and drain any remaining broth.
Meanwhile, whisk vinegar, mustard and 1/4 teaspoon each salt and pepper in a medium bowl. Slowly whisk in 4 tablespoons oil. Reserve 2 tablespoons vinaigrette in a small bowl.
Add the lentils, pecans, radishes, shallot, mint, parsley and 1/4 teaspoon salt to the dressing in the medium bowl and stir to combine.
Pat cod dry and sprinkle with the remaining 1/4 teaspoon each salt and pepper. Dredge the cod in cornstarch, pressing to adhere and gently shaking off any excess. Heat the remaining 2 tablespoons oil in a large nonstick skillet over medium-high heat. Add the cod and cook, flipping once, until it's golden brown and flakes easily with a fork, 8 to 12 minutes.
Serve the lentil salad topped with the cod and drizzled with the reserved 2 tablespoons vinaigrette. Garnish with more parsley and mint, if desired.