In a medium saucepan, combine the sugar, water, and lemon juice. Cook over medium heat until the mixture turns golden brown, about 8–10 minutes. Carefully pour the caramel into a flan mold, tilting to coat the bottom evenly. Let it cool and harden.
Place the bananas and eggs in a large mixing bowl. Blend with a hand blender until smooth and creamy, about 1 minute. Add the sugar and milk to the banana mixture. Blend again until fully combined and the mixture looks silky.
Strain the flan mixture through a fine sieve directly into the prepared caramel mold.
Fill a large saucepan with hot water and place a steaming rack inside. Bring the water to a gentle simmer over medium heat.
Cover the flan mold tightly with the flan mold lid to keep out moisture. Place it on the steaming rack, cover the pan, and steam for 30 minutes, until set. Remove the mold carefully and let it cool to room temperature, about 1 hour. Refrigerate for at least 4 hours to chill completely.
To serve, invert the mold onto a serving plate and let the caramel flow over the flan; slice into individual portions.
Notes
Watch the Caramel: I keep my eye on the pan constantly; the sugar can go from golden to burnt in seconds.Cover the Mold Well: I like to use the flan mold cover so no steam water drips inside and ruins the smooth top.Blend Everything Well: I like to blend the bananas and eggs first for a minute; it makes the base extra creamy.Strain for Silkiness: I always strain the mixture because it guarantees a velvety flan without any tiny lumps.