Wipe out the skillet, add the olive oil, and heat over medium-low heat. Add the potato and cook, stirring occasionally, until starting to soften without browning, about 7 minutes. Add the onion and cook, stirring occasionally, until the potato is cooked through, about 6 minutes longer. Add the garlic and cook, stirring, for 2 minutes. Add the spinach and chicken broth and a few grinds of pepper. Increase the heat and bring to a simmer. Immediately remove from the heat, blend in a blender until smooth, season to taste with salt and pepper, garnish with sour cream and lemon zest, and serve hot.