Preheat oven to 425 and line a muffin pan with liners
Mix dry ingredients – In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
Combine wet ingredients – In another bowl, whisk the melted butter, sourdough discard (or substitute), sour cream (or yogurt), egg, vanilla extract, and lemon zest, until smooth.
Mix the batter – Pour the wet ingredients into the dry ingredients and gently mix until just combined. Do not overmix.
Fold in the berries – Gently stir in the fresh or frozen berries. If using frozen berries, toss them in a little flour to prevent excess bleeding into the batter.
Fill muffin cups – Divide the batter evenly between the muffin cups, filling them about ¾ full. Sprinkle the tops with a little granulated sugar if desired. or Add a crumble topping (optional) – Mix 2 tablespoons flour, 2 tablespoons sugar, and 1 tablespoon butter until crumbly and sprinkle on top before baking. Bake for 8 minutes on the center rack then without opening the open door, decrease the oven temp to 350 and bake for 12 minutes or until the muffins are baked through. The tops will be slightly golden brown or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Allow to cool for 5-10 minutes before moving to a wire rack to cool.