In a food processor or blender, combine the feta, greek yogurt, zest + juice of 1 lemon and the olive oil until smooth and combined. If needed thin with just a little water. Add the herbs + a pinch of salt, pepper and the crushed red pepper flakes to the blender or food processor and pulse once more to combine.
In a medium bowl, gently combine the drained tuna with the feta sauce, the olives and sun-dried tomatoes.
Cut the loaf of ciabatta in half lengthwise, brush with a spritz of olive oil and toast.
Now spread the tuna mixture over the bottom half of the ciabatta. Layer thinly sliced cucumber, followed by the thinly shredded cabbage and arugula. Add the top half of the ciabatta and gently push down on the sandwich to help it stick together.
From here, you can either wrap the sandwich in beeswax wrap and place in the fridge for up to 1 day OR enjoy right away.
To serve, slice into 4 equal sandwiches and EAT!