In a medium bowl, whisk together olive oil, lemon juice and zest, honey, salt, and pepper until emulsified, about 30 seconds.

Rinse romaine lettuce under cold water, chop into bite-sized pieces and place in a large bowl.

Thinly slice the red onion and add it to the salad bowl.

Grate parmesan cheese and sprinkle over the salad, then add crushed red pepper flakes.

Drizzle the dressing over the salad, toss gently to combine, and serve immediately.