To make the dressing: Toast curry powder in a small pan over medium heat until fragrant, 1-2 minutes, remove from heat and transfer curry powder to a medium bowl. Wipe pan clean.
To the medium bowl, add olive oil and vinegar and whisk to combine. Puree 1/2 small banana until completely smooth, and slowly add to curry-oil-vinegar mixture, stirring vigorously to fully incorporate. Season with salt and pepper to taste and stir to combine. Set aside. Dressing will be creamy. While not necessary, the dressing can be made ahead of time to allow flavours to develop more thoroughly.
To prepare the salad: Using same pan that toasted the curry powder, toast pistachios over medium heat until fragrant or toast marks start to show, 1-2 minutes. Transfer to a small bowl, and set aside to cool.
Prepare and measure the greens, cucumbers, carrots, red onions, and bananas, and add to a large bowl. Top with toasted pistachios. Using a spatula, add just enough dressing to salad to coat (there may be extra).
Toss by hand, and garnish with remaining banana slices if desired.