Rinse the cucumber under cold water and peel if desired. Slice into 1/8-inch rounds and place in a mixing bowl.

Chop the spinach into bite-sized pieces and add to the bowl with the cucumber.

In a separate bowl, whisk Greek yogurt, lemon juice, and olive oil until smooth. Season with salt and pepper, and add chili flakes if desired.

Toast the bread slices in a toaster or on a skillet for about 2 minutes until golden brown.

Toss the cucumber and spinach with the dressing until evenly coated.

Spread the salad generously on top of the toasted bread slices and serve immediately.