Preheat grill or grill pan to medium-low and lightly brush the grates with olive oil. Season fish with salt and pepper. Arrange sliced lemons side-by-side in the center of the grill, and place fish on top. Cover grill and cook until the fish is opaque and just cooked through, about five minutes. Using a metal spatula, transfer the lemon and fish to a plate and allow to cool, then place in an airtight container.
In a separate airtight container, combine the julienned lemon, cucumber, fennel, fennel fronds, and olives.
In another airtight container, combine two teaspoons of the lemon juice with one tablespoon olive oil and the red onion. Season with salt and ¼ teaspoon aleppo pepper. Store everything in the fridge.
Shake the lemon dressing and pour over the cucumber-fennel salad. Top with the halibut.