Preheat oven to 350°F.

Cook bacon:
In large, heavy skillet over moderate heat, cook 6 slices bacon, turning occasionally, until crisp, then transfer with tongs to paper towel–lined plate to drain. Repeat with remaining 6 slices. Set aside.

Assemble sandwiches:
Slice each ciabatta in half and place on large baking sheet. Place 4 ounces turkey on each of four ciabatta bottoms, then spread onion relish over meat. Lay 3 slices bacon on each of 4 ciabatta tops. Transfer baking sheet to oven and toast until bacon is crispy and edges of ciabatta turn light golden brown, about 5 minutes (the turkey does not have to get hot). Remove from oven and transfer bacon to other side of ciabatta, laying on top of onion relish. Divide avocado slices between sandwiches, placing on top of bacon. Spread aioli evenly across each ciabatta top, close sandwiches, slice each in half, and serve.