In a large skillet, warm 1 tablespoon of olive oil over medium heat. Once the oil is shimmering, add the sliced onion. Stir occasionally and cook until the onion is soft, translucent, and aromatic, which should take approximately 3 to 4 minutes.

Place the red bell pepper, zucchini, and mushrooms into the skillet. Sauté over medium heat for 5-6 minutes, stirring occasionally, until the vegetables are soft and any excess liquid has evaporated, leaving the mixture dry and ready for the next step.

Add cumin, chili powder, salt, and black pepper to the pan. Stir well and continue cooking for 1-2 minutes over medium heat. Watch for the spices to release their aroma, which will deepen the flavor of the sautéed vegetables.

Transfer the cooked vegetable mixture, including onion, red bell pepper, zucchini, and mushrooms, from the skillet to a bowl. Use a paper towel to carefully wipe the skillet clean, removing any remaining bits. This ensures the skillet is ready for cooking the quesadillas without any residue.

Warm a skillet over medium heat and place a flour tortilla in it. Evenly distribute 1/4 cup of shredded cheese over one half of the tortilla. Spoon a generous portion of the sautéed onion, red bell pepper, zucchini, and mushrooms mixture on top of the cheese.

Sprinkle an additional 1/4 cup of shredded cheese over the sautéed vegetables, then fold the tortilla in half. Cook on medium heat for 2-3 minutes, ensuring the bottom turns golden brown and the cheese starts to melt and ooze slightly.

Gently turn the quesadilla over with a spatula. Continue cooking for another 2-3 minutes, ensuring the cheese is thoroughly melted and the tortilla achieves a golden, crispy texture. Watch closely to prevent burning and adjust the heat if necessary to maintain even browning.

Once the quesadilla is cooked, carefully move it to a cutting board. Allow it to rest for about a minute. This pause helps the shredded cheese to firm up, ensuring the filling stays intact when you slice and serve.

Continue cooking the remaining quesadillas by placing each filled tortilla in the skillet. Add a small amount of olive oil before each batch to ensure they achieve a golden, crispy exterior. Cook over medium heat for about 3-4 minutes per side, until the cheese melts.

Once the quesadillas are cooked to a golden brown and the cheese is melted, transfer them to a cutting board. Slice each quesadilla into wedges using a sharp knife. Serve promptly, adding a squeeze of lime juice over the top to enhance the fresh, zesty notes.