In a small bowl, whisk together the marinade ingredients until emulsified. Pour the marinade over the bowl of prepared veggies. Toss well to evenly distribute the marinade. Let the veggies soak up the marinade for 30 minutes or so, tossing occasionally.
De-stem the mushrooms and gently remove the gills using a spoon. Slice the mushrooms into ½-inch thick strips. Cut off the tops of the poblano peppers, slice them in half and remove the seeds and membranes. Slice the peppers into ½-inch thick strips, up to 3-inches in length. Slice off the root end and the tip of the red onion. Set it on one of its flat sides and cut down through the middle of the onion, then slice each half into ⅓-inch wide strips by slicing from one flat side to the other. Toss the portobello mushroom slices, poblano pepper slices and onion slices into a large bowl.