Quick soak the lentils by placing them in a medium-size saucepan, covering with about 2 inches of water, and bringing them to a boil. Turn off the heat and let them cool. If you prefer, you can also just put the in a bowl, cover with about 2 inches of water, and let them soak overnight.Drain the lentils well before using.
Mince the scallions, and set 1 tablespoon aside for the sauce. If desired, slice a few of the green tips into long, thin slices to press into one side of the lentil cakes (this just makes them look pretty)
Add the harissa, scallions, turmeric, cumin, garlic powder, salt, egg, egg white and baking powder to a food processor and pulse until everything is combined.
Add the drained lentils and puree until the ingredients come together as a batter and the lentils are smooth.
Add the cauliflower and half of the feta (about 1/2 cup) to the bowl and pulse about 3-4 more times to combine.
Heat a nonstick frying pan on medium-high heat, and add about 1 tablespoon olive oil.
When the pan is hot, scoop out about 2 tablespoons of the lentil batter, and add it to the pan. You should be able to cook 4 falafel cakes at time. If using the scallion strips, place a few pieces on top of each cake. Gently press each cake to flatten into about a 2-inch round.
Let the falafel cakes cook for about 3-4 minutes, and when they are set and starting to turn golden brown, flip and let them cook for another 3 minutes or so on the other side. Place them on a plate lined with a paper towel to cool. Repeat with the remaining batter.
While the falafel cakes are cooling, combine the yogurt, feta, lemon juice and zest, herbs and cucumber for the sauce. Add salt and if desired, a pinch of garlic powder to taste.
Serve the falafel cakes on top of baby greens, topped with some of the yogurt sauce.
Store leftover cakes in the refrigerator for up to 2 days, or in the freezer. Reheat in a pan or microwave before serving.