Sauté the vegetables: Chop the onion and mince the garlic. Cut the bell peppers into cubes. Heat oil in a pan over medium-low heat, then sauté the onion and garlic for 6-8 minutes until golden brown. Add the bell peppers, sweet corn, and half of the taco seasonings. If needed, season with ground cumin or spices for heat.
Cook the beef mince: Stir in the beef mince, add the remaining taco seasoning, and 1/4 cup of water. Cook for 5 minutes, until the beef is cooked through and the water has evaporated. Prepare a grill or a large pan, then add the quesadilla filling and spread it evenly. Top with grated cheese and cover with another tortilla.
Toast the quesadilla: Toast on medium-low for 2 minutes per side. Check for melted cheese, then press it down with a spatula. Once grill marks form, cut into four pieces and enjoy!