Heat 1 tablespoon oil in a large skillet over medium heat. Add sliced onions and peppers. Season with salt and pepper. Cook for 8-10 minutes, stirring occasionally, until vegetables are softened and lightly charred. Remove from skillet and set aside.
Heat 1 tablespoon oil in cast iron skillet over medium-high heat. Add diced carne asada and cook, stirring with a spatula until the liquid evaporates, about 3-4 minutes. Continue cooking until meat develops crispy brown edges, 2-3 more minutes. Season with salt and pepper to taste. Transfer to a bowl and set aside.
Wipe out the skillet and return to medium heat. Brush one side of each tortilla with melted butter. Place tortilla butter-side down in the skillet. Layer on one half of the tortilla: 1/4 cup cheddar cheese, 1/4 cup quesadilla cheese, 1/2 cup carne asada, and 1/4 cup grilled vegetables.
Fold tortilla over to create a half-moon shape. Cook for 2-3 minutes until golden brown. Carefully flip and cook additional 2 minutes until cheese is fully melted. Remove from skillet and repeat with remaining tortillas.
Cut each quesadilla into wedges. Serve with guacamole, pico de gallo, sour cream, fresh cilantro, and lime wedges.