In a blender, combine the maple syrup, miso paste, apple cider vinegar, and tamari. Blend until smooth. Reduce speed to medium and slowly add in olive oil, allowing the dressing to emulsify.
Preheat oven to 375ºF. Lightly oil a baking sheet and set aside.
Remove tough outer leaves from the Brussels sprouts, trim any discoloration from the bottom of the stem and cut the sprouts in half.
In a large mixing bowl, toss the Brussels sprouts with the maple-miso glaze until they are well coated. Scatter the sprouts over the oiled baking sheet in a single layer and roast for 25 minutes, checking and turning the sprouts sporadically. They should be soft when pierced with a fork and the glaze will be caramelized and slightly crispy. Serve warm.