Combine the tamari, brown sugar, olive oil, liquid smoke, paprika, salt, pepper, cumin and garlic powder in a 9-by-13-inch baking dish or rimmed baking sheet. Whisk to combine and set aside.

Slice the tempeh crosswise 1/4 inch thick. Arrange on a microwave-safe plate and drizzle with 1/4 cup water. Cover with a wet paper towel and microwave on high until the tempeh is soft and has absorbed some of the liquid, about 4 minutes. Drain off any excess water.

Arrange the warm tempeh slices in a single layer in the marinade. Tilt the baking dish a few times to evenly distribute the marinade. Let the tempeh sit, flipping the slices and tilting the dish every 10 minutes, until the slices are very saturated, about 30 minutes.

Meanwhile, position an oven rack in the upper third of the oven and preheat to 400 degrees F. Line a rimmed baking sheet with parchment.

Arrange the tempeh slices on the prepared baking sheet, spacing them about 1/2 inch apart; reserve the marinade. Bake until the tempeh starts to brown, about 10 minutes. Brush the tempeh with some of the reserved marinade. Flip the slices and brush again with more marinade. Bake until the tops start to char slightly, about 10 minutes.

Serve immediately or let cool completely and refrigerate in an airtight container for up to 1 week. Reheat in an oven preheated to 350 degrees F or in a dry skillet over medium heat, just until warmed through.