Preheat the oven to 425°F. Halve the red potatoes and add to a small saucepan. Cover with 1 inch water and bring to a boil. Cook potatoes until fork tender, about 12 to 16 minutes. Wipe the mushrooms with a damp paper towel and thinly slice. Peel and mince the shallot. Peel and mince 4 garlic cloves.
Place the broccoli florets onto a baking sheet, drizzle with 2 tsp olive oil, and sprinkle with salt and pepper. Roast broccoli until stalks are tender and florets are slightly browned, about 6 to 8 minutes.
Place a large nonstick skillet over medium-high heat with 1 tbsp olive oil. Add the cornstarch to a plate, and toss each piece of Sweet Earth seitan to coat; shaking off any excess. In a single layer, add the seitan to the skillet and cook until browned and crisp, about 3 to 5 minutes. Flip and continue to cook until second side is crisp, about 3 minutes more. Transfer to a plate.
Wipe skillet clean and return to medium heat with 1 tbsp olive oil. Add the sliced mushrooms and cook until they begin to brown, about 3 to 5 minutes. Add shallot and garlic, and cook until fragrant, about 1 minute. Add marsala wine and allow it to reduce for 1 minute. Add 1 cup water, bouillon cube, and reduce heat to low. Cook marsala sauce until thickened, about 5 to 7 minutes.
Once the potatoes are tender, drain, and return to the saucepan. Add the vegan butter and season generously with salt and pepper. Smash the potatoes with a fork or potato masher until they are mostly smooth. Cover to keep warm.
Return the crispy Sweet Earth seitan to the skillet with the marsala sauce and bring to a simmer. Taste sauce and season with salt and pepper. To serve, divide the smashed potatoes and roasted broccoli between large plates. Top with seitan marsala and dig in!