- Preheat your oven to 350ºF. Place apple cider vinegar, water, sugar, celery salt, salt, and pepper into a mason jar. Add sliced red onions and carrots. Seal the lid tightly and refrigerate for 2-24 hours. (The longer, the better!)
- While the veggies are pickling, whisk together balsamic vinegar, olive oil, red pepper flakes, garlic, salt, and pepper in a large bowl. Add portobello mushroom caps, and toss to coat.
- Place caps on sheet pan and roast at 350ºF for 20 minute. Reserve the marinade for another batch. Let the portobello mushroom caps cool, then slice them into thick strips. Set aside.
- Slice the hoagie buns in half lengthwise. Layer 4 slices of cucumber on the bottom, then 6-8 slices of portobello mushroom caps, and top with a handful of pickled carrots and onions. Finish with jalapeños and cilantro on top.