Preheat the over to 350 degrees F. Oil a medium-sized loaf pan (7 1/2 "x 3 1/2 " x 2 1/2").
Mash the bananas in a small bowl. Mix the stevia extract, stevia concentrate, and lemon juice into the mashed bananas. Set aside.
Sift the flour, leavenings, and salt together in a bowl.
Beat the oil and egg together in a mixing bowl until creamy. Beat in the yogurt or buttermilk and the vanilla. Stir the mashed bananas into the liquid mixture.
Fold the dry ingredients into the wet ingredients, stirring as little as possible. Mix in the walnuts just before the flour is completely blended.
Place into the loaf pan. Bake for 50 minutes to 1 hour until a toothpick or fork stuck in the middle comes out clean. Turn out the loaf and cool on a rack.
Note: For maximum flavor, use very ripe bananas (brown spotted).