To a medium saucepan, set over medium heat, add the water, along with a few pinches of salt and the Romanesco cauliflower florets. Cover the pot and simmer until the cauliflower florets are bright in color and tender when poked with a fork, about 10 minutes. Reserve the cauliflower broth by pouring it into a small bowl. Drain the cauliflower and set both aside.
To the same saucepan, add a teaspoon of olive oil. When the oil is warm, add the minced shallot and cook until soft and translucent, about 5 minutes. Add the minced garlic and cook for an additional minute. Add the reserved cauliflower florets and a few splashes of broth back to the pot. Using a hand immersion blender, pulse (going up and down) until the mixture is smooth, adding more broth as needed (I ended up adding about 1/2 cup of cauliflower broth). If you don't have a hand immersion blender, transfer everything to a food processor or blender and pulse until smooth, adding broth as needed.
Next, add the Greek yogurt, garlic powder and a few pinches of pepper. Pulse until combined. Add the juice from 1/2 lemon and salt to taste (I added about 1 teaspoon). Transfer to a bowl and garnish with more pepper and a few chopped chives.