Preheat your oven to 350F. Lightly spray or coat a square, nonstick, 8 x 8 baking dish with vegetable oil or line it with parchment. Arrange the tempeh slices in the dish.
Whisk together the vinegar, aminos or tamari/soy/water mixture, mustard, and maple syrup if using. Pour this marinade over the tempeh.
Cover the baking dish with foil and bake, covered, for 20 minutes. Remove the foil and bake for an additional 20-25 minutes, or until the marinade is completely reduced and sticky and the tempeh is dark and glazed. Serve in salads, bowls, or as a side dish with your favorite meal!