In a high-speed food processor, combine the frozen bananas, cottage cheese, lemon zest, and vanilla extract.
Process the ingredients on high for 5-7 minutes, stirring occasionally. Process until thick and smooth.* Taste the mixture and add 1 tablespoon of agave syrup or honey at a time if you prefer it sweeter. Pulse to combine.
If serving immediately, scoop the nice cream into bowls and top with crumbled graham crackers and fresh lemon zest.
If serving later, line a loaf pan with parchment paper, scoop the nice cream into the pan, and cover. Freeze until ready to serve.
Notes
Avoid over-processing the nice cream, or it may not be firm enough to scoop. If it's too thin, freeze it in a loaf pan until it firms up.