Preheat the oven to 180°C (360°F) and line an 8-inch baking pan with baking paper.
In a large mixing bowl start smashing with a fork 3 of the 4 bananas. Make sure you don't leave any big pieces.
Add the rest of the wet ingredients to the smashed bananas: oat milk, coconut oil, maple syrup, and freshly squeezed lemon juice.
Without overmixing, add the dry ingredients: oat and plain flour, lemon zest, cinnamon, baking powder and a pinch of salt.
Pour the batter into the lined baking pan and place on top the left banana divided in half-length.
Feel free to add to the mixture: chopped walnuts, blueberry, chocolate chips, vanilla or any other extra flavour that you want to have in this sugar-free vegan banana bread.
Bake the banana bread for 40 to 50 minutes (check with a toothpick if you are not sure, if this comes out clean the baked bread is ready)
Allow bread to cool in the pan for 5-10 minutes, then remove and transfer to a wire rack to finish cooling.
Once the bread is cool, cut it into 10 slices. Serve with nut butter spread on top for a healthy snack or breakfast on the go.