Heat the oil or butter in a wide skillet over medium-high heat. Add mushrooms and spread into one layer. Cook, without moving, until browned on one side, 3 to 5 minutes.

Stir the mushrooms around the pan, then add the sliced onions, peppers, onion powder, garlic powder, cumin, oregano, smoked paprika, pepper, and salt. Cook the vegetables until the onions have softened and smell sweet for about 5 minutes.

Add the kale or spinach and cook until bright green and wilted, another 2 to 3 minutes.

Make quesadillas or transfer the cooked vegetables to an airtight container and refrigerate for up to 5 days.

Heat a heavy-bottomed skillet over medium heat. Lightly spray or brush the pan with oil, and then place one tortilla into the pan.

Spread a quarter of the cheese over the tortilla and then spoon a quarter of the cooked vegetable mixture onto one half. Carefully fold the tortilla in half, and then use a spatula to press the top down to stay folded. Cook until the underside is toasted, about 2 minutes.

Carefully flip the quesadilla and cook until the second side is toasted and the cheese melts, 1 to 2 minutes. Transfer the quesadilla to a cutting board and cut in half. Repeat with remaining quesadillas.

Keep cooked quesadillas warm in a 200°F oven while you finish making the remaining quesadillas.