2 medium zucchinis, washed and dried
1 medium white or Yukon Gold potato, washed and dried
1 large egg
1 tablespoon chopped parsley
1 teaspoon grated lemon zest
1 tablespoon fine breadcrumbs, plus more as needed if mixture is "soupy"
8 tablespoons unsalted butter, divided
Sour cream, for serving, may substitute whole-fat plain Greek yogurt