Using a food processor or box grater, coarsely shred the zucchini and green apple. If you notice any large strips of apple peel, remove and discard them. Toss with 1 tablespoon of granulated sugar and a pinch of salt. Place in a fine mesh strainer set over a bowl, allowing excess water to drain from the zucchini and apple for at least 20 minutes.

In a large bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, cinnamon, salt, nutmeg, ginger, cloves, and allspice. Set aside.

In a liquid measuring cup or small bowl, whisk together the milk, sour cream, cooled melted butter, eggs, and vanilla, until combined, about 15 seconds. Set aside.

Using paper towels or cheesecloth, firmly squeeze the zucchini and apples to wring out any excess moisture. Break apart the squeezed zucchini and apple with your hands before adding them to the flour mixture. Pour in the wet ingredients and stir with a spatula until just combined. Cover and let batter rest for 20 minutes.

Meanwhile, prepare the crumble. In a small bowl, add the flour, oats, pepitas, brown sugar, cinnamon, and salt. Using a pastry blender, fork, or your fingertips, press the butter into the dry ingredients until the mixture forms crumbly clumps.

Divide the batter evenly among the 12 liners, filling each almost to the top (about a heaping 1/4 cup each). Sprinkle a heaping tablespoon of crumble over each muffin, and gently press into the batter.

Cool muffins on a rack for 10 minutes before removing from the muffin tin. Let muffins cool completely.