In a medium bowl, stir together garlic, scallions, sesame seeds, sesame oil, soy sauce, vinegar, sugar, and pepper flakes if using.
Roughly chop spinach into large pieces. Add a thin layer of olive oil to a large skillet over medium heat. Add spinach and cook until bright green and wilted, 1 to 2 minutes. Use tongs to transfer spinach to the bowl with sauce, and toss to coat.
Let cool for 5 minutes, then taste and adjust seasonings as desired.
