In a small bowl, mix together the chopped red onion and sliced tomatoes. Add a squeeze of lime, salt and pepper. Stir and set aside.
Preheat your grill or grill pan. In another small bowl, mix the marinade ingredients together (olive oil, balsamic, mustard, salt & pepper), and brush onto the portobello slices until they’re coated.
Grill mushroom strips on each side until grill marks form and mushrooms are tender and juicy (about 3-4 minutes per side). Place 2 mushroom slices into each hot dog bun. Top liberally with onion & tomatoes, some serrano slices, and a swirl of mustard.