For the jackfruit brisket: Preheat the oven to 300 degrees F. Line a baking sheet with parchment paper and spray with nonstick cooking spray.
Place the vital wheat gluten in a large mixing bowl. Whisk in the paprika, salt, chili powder, granulated garlic, ground mustard, onion powder, black pepper and sage. Add in the jackfruit and mushrooms. Stir until incorporated. Use your hands to break up any large clumps and make sure the mixture is thoroughly mixed; it will resemble a dark streusel with pea-size pieces.
Whisk together the stock and aminos in a small bowl. Pour this liquid into the jackfruit mixture. Use your hands to aggressively knead the mixture until it feels/looks like seasoned ground meat.
Place the mixture onto the prepared baking sheet. Use your hands to press it into the shape of a brisket, about 1-inch thick. Cover the baking sheet with aluminum foil and bake until firm to the touch, about 1 1/2 hours.
For the BBQ sauce: Meanwhile, combine the ketchup, beer, brown sugar, mushroom stock, vinegar, tomato paste, smoked paprika, onion powder, chili powder and salt in a saucepot. Place over medium-high heat and whisk until simmering. Reduce the heat to low and cook until thick, 15 to 20 minutes more. Set aside.
Remove the aluminum foil and spread the BBQ sauce over the top, reserving some for serving. Turn the oven to broil on high. Place the "brisket" back into the oven and broil until the sauce bubbles and caramelizes, 2 to 3 minutes more.
Remove from the oven and allow to cool for 10 minutes. Slice into thin pieces and serve warm with more BBQ sauce.
Pulse the mushroom stems and caps in a food processer to get them finely chopped.
Store-bought BBQ sauce can also be used.