In a medium bowl, whisk together the olive oil, lemon juice, lemon zest, honey, salt, and pepper until well combined and slightly emulsified.

In a large salad bowl, combine the chopped Romaine lettuce, sliced red onion, and grated Parmesan cheese. If desired, add the crushed red pepper flakes.

Drizzle the prepared dressing over the salad mixture and toss gently until the Romaine is well coated.

Transfer the salad to a serving platter or individual bowls and serve immediately for best freshness.

Store any leftovers separately from the dressing in an airtight container in the refrigerator, and consume within a day.