Preheat oven to 350°F. Prepare a 12-cup muffin pan with liners or nonstick cooking spray. Set aside.
In a large mixing bowl, whisk together the almond flour, baking powder, ground flaxseed, and salt until well combined. Set aside.
In a separate large bowl, use a hand mixer to cream together the butter and brown sugar substitute until light and fluffy. Set aside.
In a large measuring cup, whisk together the eggs, half-and-half, and vanilla extract.
Add the butter mixture to the egg mixture. Beat on low speed until well combined—mixture may look slightly curdled before it becomes smooth.
Gradually add the dry ingredients to the wet mixture, stirring gently after each addition, until a smooth batter forms.
Fold in the mashed bananas and walnuts, if using.
Divide the batter evenly among the muffin pan cavities, filling each about 2/3 to 3/4 full. A cookie scoop works well for portioning.
Bake for 18-20 minutes, checking for doneness at 18 minutes. Muffin tops should be golden and a toothpick inserted into the center should come out clean.
Let the muffins cool in the pan for 10-15 minutes. Transfer to a wire rack to cool completely before serving, 30-45 minutes.