Sauté Mushrooms to Golden: Warm the oil or butter in a large skillet over medium-high heat. Spread the sliced mushrooms evenly in the pan and let them cook undisturbed for 3 to 5 minutes, allowing them to brown deeply on one side for rich flavor.
Add Aromatics and Peppers: Stir the mushrooms gently, then add the sliced onion and bell or poblano pepper. Sprinkle in the onion powder, garlic powder, cumin, oregano, smoked paprika, black pepper, and salt. Cook the mixture, stirring occasionally, until the onions soften and release their sweetness—about 5 minutes.
Wilt the Greens: Toss in the torn kale or spinach and continue cooking until the greens turn bright and wilt down, about 2 to 3 minutes. Remove the vegetable mixture from heat.
Assemble Quesadillas: Heat a clean heavy skillet over medium heat and lightly grease it with cooking spray or oil. Place one tortilla flat in the pan. Sprinkle a quarter of the shredded cheese evenly over the entire tortilla. Spoon about a quarter of the cooked vegetable mixture over half of the tortilla. Fold the other half over the filling, pressing gently to seal.
Cook Until Crispy and Melted: Allow the folded quesadilla to cook for approximately 2 minutes or until the underside is golden and crisp. Carefully flip it using a spatula and cook the other side for 1 to 2 minutes until the cheese inside melts completely and the tortilla is toasted.
Serve and Repeat: Transfer the cooked quesadilla to a cutting board and slice into halves or wedges. Keep finished quesadillas warm in a low oven (around 200°F) while you prepare the rest. Repeat the assembly and cooking process with the remaining tortillas, cheese, and vegetables.