Thoroughly wash romaine lettuce heads and pat completely dry with paper towels. Keeping the core intact, cut each head lengthwise into two halves.
Preheat your grill or grill pan to medium-high heat (400-425°F).
Brush the cut sides of each romaine half generously with olive oil and season with salt and freshly ground black pepper.
Place the romaine halves cut-side down on the hot grill. Let them cook undisturbed for 2-3 minutes until nicely charred but still crisp.
Using tongs, carefully flip the romaine halves and grill for an additional 1-2 minutes just to warm the exterior.
Transfer the grilled romaine to a serving platter, arranging them cut-side up to display the char marks.
While still warm, drizzle with fresh lemon juice, then sprinkle evenly with grated Parmesan cheese.
Add any optional toppings of your choice.
Serve immediately while the contrast between warm charred edges and cool centers is at its peak.