Meanwhile, warm the olive oil in a small skillet over medium heat. Add the garlic and cook until the garlic begins to sizzle, then reduce the heat to low. Cook gently, lightly shimmying the pan and turning the garlic every now and then, until the garlic is soften and turning lightly golden, about 5 minutes. Remove the skillet from the heat and pour the contents into a stand blender.
Add the cooked kale stems to the blender when the time’s up. Use tongs to transfer the kale leaves over to the blender (you actually want to bring some of the water with them). Add most of the zest from the lemon, and 1 tablespoon of the juice. Add ¼ teaspoon salt, the red pepper flakes (if using), plus about 10 twists of freshly ground black pepper.
Blend until the mixture is completely smooth and silky, adding more spoonfuls of water if necessary to gain traction. Taste, and add more salt, pepper or pepper flakes, and/or another tablespoon of lemon if desired. We want it to taste lemony! Set aside.
