Combine the lentils, garlic and 4 cups water in a large heavy pot. Bring to a boil and reduce to a simmer. Simmer until the lentils are soft, about 15 minutes. Drain the lentils, discarding the garlic cloves, and set aside to cool to room temperature.
In a mixing bowl, whisk together the lemon juice and olive oil until combined. Add the tomatoes, scallions, basil, dill, corn and cooled lentils. Season with salt and pepper. Add the lemon zest and toss to combine, making sure everything is evenly incorporated.
Serve cold or at room temperature. This dish can be made 1 day ahead.