Preheat oven to 425°F.
In a small bowl, whisk together balsamic vinegar, dijon mustard, maple syrup, garlic, salt and pepper.
Place portobellos in a large resealable plastic bag. Pour in marinade, seal bag, and toss to coat mushrooms thoroughly. Let marinate at room temperature for 30 minutes.
Place mushrooms onto a foiled lined sheet and roast for 10 minutes. Drain liquid from the pan, flip mushrooms and continue to roast for 10 additional minutes.
In a skillet, heat 2 tbsp of olive oil over medium heat until oil is shimmering. Add onions and stir to coat with oil. Spread onions evenly in the pan and cook on low-medium heat for 35-40 minutes until browned. Sprinkle with salt and remove onions from pan.
Mix shredded veggies. In a small bowl, combine apple cider vinegar, mayo, mustard, agave, celery seed, and salt. Toss veggies with slaw dressing.
To serve, slice open a pretzel bun. Layer sweet baby lettuces and portobello mushroom on top of the bottom bun. Top portobello mushroom with slaw and caramelized onions. Enjoy!