To make the maitake mushrooms, break the mushrooms into smaller pieces and place on a perforated tray. Transfer to a smoker and smoke the mushrooms for 2 hours according to the manufacturer's instructions.

Preheat the oven to 375°F. Line a baking sheet with parchment paper.

In a large bowl, combine the smoked mushrooms, olive oil, salt and pepper and toss well to coat. Transfer the mushrooms to the prepared baking sheet and roast until golden and slightly crispy, 10 to 15 minutes. 

To make the barbecue sauce, in a saucepan over medium heat, warm the olive oil. Add the onions and cook, stirring occasionally, until just translucent, about 5 minutes. Add the garlic, miso, mustard, ginger juice, maple syrup, chipotle chile, vinegar, water, salt and pepper. Bring to a simmer and cook, stirring occasionally, for 15 minutes. Remove from the heat. 

Transfer the mixture to a blender and blend until smooth. Strain the sauce through a fine-mesh sieve.

To make the crostini, preheat the oven to 350°F.

Cut the baguette on the bias into 1/2-inch slices. Brush or drizzle the slices on both sides with the olive oil. Place on a baking sheet and bake until golden, 15 to 20 minutes, rotating the pan halfway through to ensure even cooking.  
 

To serve, place the roasted mushrooms in a large bowl. Pour the miso barbecue sauce over the mushrooms, season with salt and pepper and toss to combine.  

Place 2 crostini on each plate. Mound the mushrooms on the crostini and top with the micro arugula. Serve warm.