Slice the ciabatta loaf in half lengthwise. Pull out some of the top slice to make space for the fillings. Brush the cut sides of both halves with 2 tablespoons extra virgin olive oil.
Heat broiler to high. Place the bred halves, cut side up, and toast for 1-2 minutes until golden to your liking. Remove and set aside.
In a small bowl, stir together the mayonnaise and pesto until combined.
Spread about half of the pesto aioli on the bottom slice of the ciabatta. Top with the cheese slices. Lightly mash the avocado and spoon over the cheese. Arrange the tomato, cucumber, and pickled onions. Top with the lettuce. Place the top piece onto the fillings and carefully slice into 4-8 sandwiches (4 big bois, 8 smol bois). Serve immediately.
Notes
*To make ahead: Omit the avocado and add when serving to prevent browning. The sandwich otherwise will keep up to 3 days fully assembled. Ciabatta is sturdy and should not get soggy from the filling.*Pickled red onions: Thinly slice a small red onion and place into a jar. In a small bowl, stir together 1/2 cup white vinegar, 1 tablespoon sugar, and 1 teaspoon salt. Bring 1/2 cup of water to just a boil, then pour into the bowl with the vinegar mix and stir until the granules dissolve. Pour over the onions to completely cover. Seal and let sit 30 minutes, or up to 5 days in the fridge.