Preheat oven to 350°F. Coat an 9-by-5-inch loaf pan with baking spray; set aside. Finely mash strawberries in a large bowl with a potato masher. Add egg, 2/3 cup brown sugar and 1/4 cup melted butter; whisk to combine. Add all-purpose flour, 1/2 cup whole-wheat flour, 1 teaspoon cinnamon, baking powder, 1/2 teaspoon salt and lemon zest; stir until well combined. Spoon the batter into the prepared pan.