Slice the block of tempeh into two-inch strips.
In a bowl, mix together the soy sauce, lime juice, sugar, red pepper flakes, and powdered garlic.
Place the tempeh slices in a flat container (like a casserole dish) and pour the mixture over it. Leave it to marinate for 20 minutes, turning the pieces over after 10. Save the marinade.
After the 20 minutes are up, heat the oil in a frying pan and add the tempeh (not the marinade!). Fry on all sides until brown.
In a small bowl, combine the marinade mixture and three teaspoons of peanut butter. Using a fork, mix it together until all the liquid is soaked up into the peanut butter.
Lightly toast the whole wheat buns. Spread some of the peanut butter mixture on them. Add some spinach leaves. Take the fried tempeh and cut it in half to make it flatter and easier to bite! Add that to the sandwich and enjoy!