Preheat oven to 350°F/177°C.
Grease a small-to-medium sized ramekin with coconut oil.
Place a banana, almond flour, fine polenta, lemon zest, and an egg yolk in a big bowl and mix. Stir in butter and lemon juice. Slowly add almond milk to batter and mix gently.
Place an egg white in a clean bowl, add a pinch of salt and whisk until soft peaks are formed. Gently fold the egg white into the lemon mixture. Carefully pour the mixture into the greased ramekin.
Place the ramekin gently into a deep baking pan/tray and place in the oven. Carefully fill the baking pan/tray with hot water to about halfway up the side of the ramekin. Bake until browned on top, about 25-30 minutes.
Leave to cool for about 10 minutes before serving.