Heat oven to 350°F. Grease a 9×5-inch loaf pan and line with a parchment sling.
Whisk flour, baking soda, baking powder, cinnamon, and salt in a medium bowl.
Mash bananas in a large bowl until mostly smooth.
Whisk in sweetener, oil, eggs, applesauce, and vanilla until well blended.
Sprinkle dry ingredients over the wet mixture. Stir gently just until no dry streaks remain.
Assess thickness; if very thick, stir in the milk. Fold in walnuts, if using.
Scrape batter into the prepared pan and smooth the top.
Bake 50–60 minutes, until a toothpick comes out clean or with a few moist crumbs.
Cool 10 minutes in the pan, then lift out and cool completely on a rack before slicing.