Start by preparing the pickled cucumbers. In a large glass jar, stir together the julienned cucumber, filtered water, vinegar, maple syrup, and 1/4 teaspoon of the sea salt. Secure the lid, and shake for 10 to 15 seconds to mix. Refrigerate for at least 30 minutes, or until ready to assemble.
In a large resealable bag, whisk together the smoked paprika, onion powder, garlic powder, the remaining 1/4 teaspoon sea salt, and the coconut sugar. Add the sliced mushrooms, seal the bag, and toss to coat.
In a large nonstick skillet, heat the olive oil over medium heat. Add the garlic and sauté for 30 seconds, or until just beginning to soften. Add the seasoned mushrooms and cook for 5 minutes, stirring frequently.
In a medium bowl, whisk together the barbecue sauce and water. Pour over the mushrooms, cover, reduce the heat to medium-low, and simmer for 10 minutes, or until firm-tender, stirring occasionally. Remove the lid and continue to cook for another 5 minutes, or until most of the liquid has absorbed and the mushrooms lose most of their glossy sheen, stirring occasionally. Season with more sea salt to taste.
Meanwhile, heat a small skillet over medium-high heat and warm the tortillas, one at a time, for 10–20 seconds on each side, or until softened. Wrap in foil to keep warm until ready to serve.
Spoon a bit of the BBQ portobellos into each warm tortilla. Drizzle with more barbecue sauce, to taste. Top with the pickled cucumbers, scallions, cilantro (if using), and sesame seeds (if using).