Grease a 9" x 5" loaf pan and set aside. Preheat oven to 350 degrees F (180 degrees C).
Heat Unsalted Butter (1/4 cup) over medium heat in a small saucepan, stirring occasionally, until little brown solids form in the bottom of the pan, about 3 minutes. Remove from heat and immediately transfer to a large bowl.
Add Unsweetened Apple Sauce (1/4 cup) and white Granulated Sugar (1 1/4 cup) and beat together using an electric mixer until thoroughly combined. Then blend the Egg (2) into the mixture.
In a small cup, combine Lemon Juice (1/2 tablespoon) and Milk (1/2 cup) and let rest for 2 minutes until it curdles.
Mash the Banana (1 1/2 cup).
Pour milk mixture in to the butter mixture, add mashed banana, and beat on low until smooth.
Then, add Vanilla Extract (1 teaspoon), All-Purpose Flour (2 1/2 cup), Salt (1 teaspoon) and Baking Soda (1 teaspoon), mixing on low until well combined.
Pour 1/2 cup of banana batter in to a separate, small bowl, and add Fresh Raspberries (1 cup). Beat on high with mixer until smooth.
Pour banana batter in to the greased loaf pan, then add dollops of raspberry batter to the pan. Swirl the batter with a knife, being careful not to scrape the bottom of the pan, until a marbled effect is achieved.
Bake for a total of 1 hour 15 minutes. 30 minutes into the baking, loosely drape a sheet of aluminum foil over the top of the bread to prevent it from burning.
Cool for 10 minutes in the pan and then transfer to a wire rack to cool completely.