Preheat your oven to 400°F and line a large sheet pan with parchment paper for easy cleanup.
Cut the seitan into bite-sized 1-inch cubes and dice the red potatoes into similar sizes to ensure they cook at the same rate.
In a large bowl, combine the seitan, potatoes, chopped bell peppers, and sliced red onion.
Drizzle with extra virgin olive oil and add the minced garlic, chopped rosemary, sea salt, black pepper, and smoked paprika.
Toss everything thoroughly until the spices and oil are evenly distributed across all the ingredients.
Spread the mixture onto the prepared baking sheet in a single layer, making sure not to overcrowd the pan.
Roast for 25 to 30 minutes, flipping the ingredients halfway through, until the potatoes are fork-tender and the seitan has developed a satisfyingly crispy exterior.
Remove from the oven and serve immediately while hot and fragrant.