In a mixing bowl, whisk together the eggs, Parmesan (if using), salt, and black pepper.

In a skillet over medium heat, sauté the spinach until just wilted. Stir the spinach into the egg mixture.

Stovetop Method: Heat olive oil in a non-stick pan over medium heat. Pour in the egg mixture and cook, covered, until the edges are golden. Flip carefully and cook until set.

Oven Method: Preheat oven to 400°F (200°C). Heat olive oil in an ovenproof skillet, add the egg mixture, and cook for 5–8 minutes. Transfer to oven and bake 15–20 minutes until fully set.

Mash the avocado with lemon juice, salt, and pepper to create avocado spread.

Slice ciabatta rolls and toast them lightly.

Assemble the sandwich: Spread avocado mash on the top half of each ciabatta. Layer the bottom half with lettuce, slices of frittata, and sun-dried tomatoes. Close with the avocado side and serve warm.