Place the water, vinegar, vegetable oil, sugar, garlic, ginger, crushed red pepper flakes and tamari in a small saucepot over medium-high heat, bring up to a bubble and simmer one minute. Remove and discard the garlic and ginger.
Place the sliced onions in a salad bowl and pour the hot mixture over them. Let sit until cool.
Add the cucumbers and spinach to the salad bowl, toss to combine and season with salt and pepper to taste.