2 tablespoons olive oil
1 medium leek, white and pale-green parts only, finely chopped
1 garlic clove, thinly sliced
1 tablespoon tomato paste
1 cup green lentils, preferably French
2 tablespoons reduced-sodium soy sauce
Kosher salt, freshly ground pepper
Thinly sliced scallions (optional; for serving)
Heat oil in a medium saucepan over medium heat. Add leek, garlic, and tomato paste and cook, stirring often, until fragrant and tomato paste begins to darken, about 4 minutes. Add lentils and 2 1/2 cups water. Bring to a boil; reduce heat, cover, and simmer, stirring occasionally, until lentils are tender, 45–55 minutes.
Remove from heat and let sit, covered, 10 minutes; add soy sauce and season with salt and pepper. Serve lentils topped with scallions, if desired.
DO AHEAD: Lentils can be made 5 days ahead. Cover and chill.