If you haven't yet, grate your summer squash with a coarse grater and set aside. If using frozen, ensure it's thawed completely before beginning. You don't need to completely squeeze all the liquid out of the squash, but we don't want it to be dripping. I recommend just doing 1-2 squishes to remove excess water, especially if it was previously frozen.
Preheat the oven to 350°F and grease a 9x5 loaf pan. Using the spent butter wrapper works great.
Whisk the dry ingredients (flour, baking powder, baking soda, salt, cinnamon, and lemon zest) together to blend. In a separate bowl, combine your wet ingredients (butter, sugar, brown sugar, eggs, lemon juice, sour cream or yogurt, and vanilla).
Pour the wet ingredients into the dry ingredients, and stir until barely combined. Then fold in your squash and nuts. Stir until there are no more flour streaks, and then stop.
Pour into the loaf pan and bake for ~1 hour. Test with a toothpick, poking it into the center and ensuring it comes out clean before removing from the oven. Leave in for additional 5-minute increments if the toothpick comes out completely gooey. It should reach an internal temperature of 200℉ in the center.
Allow the loaf to cool in the pan for 10-15 minutes, then loosen around the sides with a butter knife. Then carefully flip out onto a cooling rack. Allow to cool completely before slicing or glazing.