For the olives, you want a variety that is fleshy, easy to pit, and not overpoweringly salty. Alphonsos are a nice choice. Whatever you do, don't use the tasteless canned "Ripe California" olives. If you only have the very salty kind, pit and soak them for ten minutes in hot water, then squeeze them out.
In a mini-food processor, thoroughly puree the olives and garlic. Add the lemon zest and juice and process for 20 seconds. Add the 1/4 cup of olive oil, 2 teaspoons at a time, processing for 15 seconds after each addition to emulsify. Let rest and then taste and adjust acid and salt before serving. You want it at room temperature for serving.
Heat a grill pan over a medium-high flame. Brush the bread with olive oil and toast on each side until nicely browned and marked by the grill. Push down a little to get nice marks.
Brush the cut side of the romaine and grill for about 30 seconds, pushing down gently.
To serve, put each piece of bread on a plate. Top with two romaine halves, some of the cucumber, tomato, red onion, and the parmigiano. Drizzle on the dressing and finish with a grind of black pepper.