King Ranch Chicken Tostadas

In a large skillet, heat oil over medium-high. Add onion and cook 4 minutes. Add mushrooms and cook, stirring occasionally, until onion is softened and mushrooms are golden brown, about 6 minutes. Season with salt and pepper. Add tomatoes with chiles and cook, stirring, until warmed through, about 2 minutes.

Spread tomato mixture on tostada shells. Top with chicken, lettuce, cheese, and sour cream.