If you're using wooden skewers, place them into a large container such as a baking pan and submerge them in water. Let them soak until ready to use.
Combine the lemon juice, olive oil, soy sauce, garlic, sugar, paprika, oregano, thyme and black pepper in a small container.
Place the seitan pieces in a shallow dish, then pour the lemon juice mixture over them. Let the seitan marinate for about 30 minutes, stirring occasionally to coat the seitan in the marinade (Note 1).
Preheat your oven to 425°F or, if grilling, lightly oil your grill and build a medium fire.
Skewer the seitan pieces, alternating with pieces of the red and green peppers and the red onion. Lightly brush with the marinade.
If baking: Arrange your skewers on a parchment paper-lined baking sheet, then bake them for about 15 minutes, turning and generously drizzling with any leftover marinade about halfway through, until the seitan and veggies are tender and darkened around the edges.
If grilling: Grill until the seitan and veggies are tender and lightly charred, about 10 minutes, turning halfway and generously basting with leftover marinade.
Serve.