This Mexican classic is commonly served topped with eggs, but it's just as delicious without, as a savory crowd-pleaser that skews more toward the lunch end of brunch. Simply bake the casserole, top with cheese, and serve.
Heat oil in a large skillet over medium-high heat. Add chorizo; sauté 5 minutes or until browned. Remove from skillet with a slotted spoon, and drain on paper towels. Add onion and next 5 ingredients to skillet. Sauté 5 minutes or until vegetables are slightly soft. Add drained chorizo, chipotle, salt, and tomatoes. Bring to a simmer; cook 5 minutes or until liquid thickens slightly. Remove from heat; let cool 15 minutes. Spread 1/2 cup tomato mixture into the bottom of a 9- x 13-inch baking dish. Arrange 6 tostadas over tomato mixture. Top with half of remaining tomato mixture. Repeat with remaining tostadas and tomato mixture. Cover and refrigerate 8 hours or overnight. Preheat oven to 425°. Bake, uncovered, 35 minutes. Remove baking dish from oven, make 8 shallow indentations in the casserole with the back of a spoon, and carefully crack in eggs. Bake 5 minutes or until egg whites are just set. Remove baking dish from oven, and preheat broiler to high. Sprinkle casserole with queso fresco; broil 2 minutes or until cheese melts. Top with avocado and cilantro, and serve with lime wedges.