Set the oven to Broil.
Into a large skillet over medium heat, add the olive oil. When the oil is hot, add the onion. Cook for 2 minutes, stirring frequently until tender. Add the garlic and continue cooking for an additional 1 minute.
Add the grated zucchini, lemon zest, salt, and pepper. Toss to combine with the onion mixture. Remove from heat and set aside to cool.
Onto a baking sheet, arrange the baguette slices in an even layer. Set in the oven just long enough to lightly toast the bread.
Spoon the cooled zucchini bruschetta onto each piece of bread. Garnish with red pepper flakes, if using, and serve.