For additional crunch, top each muffin with about 1 tablespoon of granola before baking.
Preheat oven to 400 degrees. Line a standard 12-cup muffin tin with baking cups, or lightly brush with oil. In a medium bowl, whisk together almond butter, mashed bananas, eggs, sugar, salt, baking soda, and vinegar until just combined.
Divide batter evenly among muffin cups; top each with 2 to 3 banana slices. Bake until golden and tops spring back when lightly touched, about 18 minutes. Let cool 5 minutes in tin, then transfer to a wire rack and let cool completely.