Cover lentils with 2 inches cold water. Soak 4 hours at room temperature or up to 1 day in refrigerator. Drain well. Pulse pale-green and white parts of scallions in a food processor until minced. Add harissa, turmeric, baking powder, 1/4 teaspoon salt, and egg and egg white; pulse to combine. Add lentils; puree until almost smooth. Transfer mixture to a bowl, and stir in a little more than half the feta.
Whisk remaining feta, the yogurt, lemon juice, and garlic in a small bowl. Combine cucumber, cress, and mint in another bowl.
Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Place a scant tablespoon of scallion greens in pan, and spoon about an eighth of the batter on top, spreading to make a 3-inch cake. Repeat, making 4 cakes at a time. Cook until golden brown, flipping once, about 6 minutes. Add remaining teaspoon oil, and repeat.
Divide sauce among 4 plates; top with salad. Place cakes next to greens, and serve.
Peppery cress is rich in beta-carotene, vitamin C, and other powerful cancer-fighting compounds.