Combine all marinade ingredients in a ziplock bag and mix well so that the portobello mushrooms are well coated in olive oil and blackening seasoning. Let sit in the refrigerator for at least 1 hour.
Heat a pan over medium heat. Add portobello mushrooms. Cook until they are oozing liquid and blackened to your liking, at least 20 minutes. Flip them every 5 minutes. When done, turn off the heat. Alternatively, you can grill these instead.
Take the 2 pita pockets and slather minced garlic inside each pocket. Then heat the pita pockets on a pan over medium-low heat, about a few minutes per side.
Assemble the pita sandwiches: add portobello mushrooms, spinach, and shredded cheese to the pita pockets. Drizzle with sriracha, if desired. Serve immediately.