Preheat your oven to 400°F and line a baking sheet with parchment paper.
Dice the red potato into 1/2-inch cubes and toss with 0.5 tbsp of olive oil, sea salt, and black pepper.
Spread the potatoes evenly on the baking sheet and roast for 20-25 minutes until the edges are golden and crispy.
While the potatoes roast, slice the seitan into bite-sized strips and thinly slice the mushrooms and onions.
Heat the remaining 0.5 tbsp of olive oil in a large skillet over medium-high heat.
Add the seitan to the skillet and sear for 3-4 minutes until it develops a savory, browned crust.
Add the mushrooms and onions to the skillet, sautéing for 5-6 minutes until the mushrooms have softened and turned a deep golden brown.
Stir in the minced garlic, dried thyme, and tamari, cooking for an additional 2 minutes to allow the flavors to meld.
Remove the roasted potatoes from the oven and toss them into the skillet with the seitan mixture to combine before serving.