Preheat a large pan over medium-high heat. Sauté the onion in the olive oil with a big pinch of salt for about 5 minutes, until lightly browned. Add the seitan and cook for 7 to 10 minutes, until browned. Add the garlic and black pepper and sauté for about a minute, until fragrant. Add a tiny bit more oil if needed.
Mix in the broccoli and cover the pan to cook, lifting intermittently to stir, until softened, about 5 minutes.
While that’s cooking, mix all of the BBQ sauce ingredients together in a bowl.
Add the sauce, toss to coat, and cook for about 5 more minutes. The sauce should become thick and bubbly. Taste for salt and seasoning and serve.