Preheat the oven to 425°F/220ºC. Arrange a rack in the middle of the oven.Option 1: line muffin tins with muffin liners.Option 2 (my preference): Generously grease a standard 12-cup muffin tin on the bottoms and sides with oil or nonstick cooking spray—be generous to avoid sticking. Rub your oiled hands on top of the tin as well to ensure the muffin tops don’t stick to the pan. Stir the lemon juice into the oat milk and set aside for 5 to 10 min. This is the vegan "buttermilk."
Add the bananas to a medium bowl and mash with a fork until smooth and no lumps remain. Transfer to a cup or plate so you can wipe out the bowl and use it in step 6.
Make the Walnut Crumble: In a small bowl, mix the finely chopped walnuts, brown sugar cinnamon, and pinch of salt until well combined.
In a large mixing bowl, add the brown sugar and oil. Mix using the electric mixer or a whisk until well combined. Set aside for a minute.
Shake the can of chickpeas to evenly distribute the starches. Measure ½ cup (120 mL) of the aquafaba into the bowl used for the bananas. Using a handheld electric mixer, whip the aquafaba on medium speed for 1 minute until uniformly foamy.
Add the whipped aquafaba into the sugar-oil mixture, and mix until well incorporated. Add in the vegan "buttermilk" and vanilla and mix until smooth. Add the mashed bananas and mix until well incorporated.
Add the flour, salt, baking soda, cinnamon, and nutmeg to the wet ingredients. Switch to using a silicone spatula (or a wooden spoon). Gently stir until the ingredients are just barely combined. It’s okay if you can see a few traces of flour.
Evenly divide the batter between the muffin tins, filling to the top. Sprinkle the Walnut Crumble evenly over each muffin.
Add the muffin pan to the oven and bake at 425ºF/220ºC for 6 minutes, or until muffins have risen nicely. Then, without opening the oven door, reduce the heat to 350ºF/175ºC. Bake for another 18 minutes, or until a toothpick comes out clean or with a few crumbs. Transfer to a cooling rack and cool for 20 minutes. Use a small offset spatula or butter knife to slide underneath the muffin tops. Once the tops have been released, you can lift the muffins up and out of the pan with your hands. Store cooled muffins in an airtight container on the counter for 2 to 3 days, or in the fridge for 5 to 6 days, or up to 3 months in the freezer.