Spicy cucumber side dish (Oi-muchim: 오이무침)

Ingredients (Serves 4)

1 English cucumber (or 2 to 3 kirby cucumbers), washed
1/4 cup thinly sliced onion
1 green onion, chopped
2 garlic cloves, minced
2 tablespoons soy sauce
2 teaspoons hot pepper flakes
2 teaspoons sesame oil
2 teaspoons sesame seeds
1 teaspoon sugar (optional)