In a medium bowl, whisk together olive oil, vinegar, oregano, and Essential Blend seasoning. Add spinach and toss to combine. Set aside.
Lightly oil both sides of each Portobello mushroom. Heat griddle to medium and cook mushrooms on both sides, flipping every 2 minutes. Use a large griddle press to flatten mushrooms. Season both sides with High Plains Steak seasoning during the last few minutes of cooking.
While mushrooms are cooking, mix smashed feta, Cholula hot sauce, and mayo in a small bowl.
To assemble burger: spread spicy feta mixture on both sides of toasted buns. Add spinach, 2 Portobello mushroom caps, sliced tomato, and sliced onion to bottom bun, then top with the other bun.
