Peel and grate the cucumber. Put the grated cucumber into a strainer and set in the sink for 10 minutes to drain.
Squeeze the cucumber to remove excess moisture and place into a small bowl. Add the remaining ingredients and stir well to combine. Cover and place in the fridge to let the flavors meld while you cook.
Heat the oil in a pan over medium heat. Add the seitan bits and garlic and stir until the seitan begins to brown. Add all of the seasonings and stir well. Sautee until the seitan is well cooked and fragrant. Turn off the heat.
Assemble the sandwich with the shredded lettuce, diced tomatoes and onions, cooked seitan and cucumber sauce. Eat open-face or fold up and hold on tight!