Preheat oven to 180ºC (350ºF), and line a baking tray with baking paper.
Prepare the mushrooms by cutting the caps (tops) and the very bottom off, which is a little woody (toss this bit out). Thinly slice the tops, then shred the rest with a fork. Start by making deep grooves into the mushroom on all sides, then start to shred it with your hands (see the video above for exactly how it's done).
Combine the olive oil, smoked paprika, cumin, chipotle, salt, and pepper in a small bowl. Mix to combine, then add to the shredded mushrooms and toss gently to coat well.
Transfer mushroom mix to the prepared baking tray and cook for 15 minutes, or until the mushrooms have softened and shrunk.
Make the sauce while the mushrooms cook. Add the tomato passata, maple syrup, apple cider vinegar, dijon mustard, salt and pepper to a small saucepan. Stir to combine, bring to a boil then simmer for 15 minutes. The sauce should have reduced and be quite thick by this time.
Remove mushrooms from oven, add to a bowl and spoon over half the sauce. Toss to combine. Add more sauce to your liking if needed and toss through again.
Heat your tacos as per packet instructions then assemble. Start by adding the mushrooms then anything else you'd like on top. Enjoy!