How to Make It
Heat olive oil in a large skillet over high. Add onion, chipotle, oregano, crushed red pepper, and garlic. Cook, stirring occasionally, until onions are translucent, 6 to 8 minutes. Place onion mixture in a 6-quart slow cooker; stir in tomatoes. Cover and cook until juices have released from fresh tomatoes and their flesh is broken down, about 4 hours on HIGH or about 8 hours on LOW.
While the tomato mixture is cooking, preheat oven to 350°F. Coat both sides of corn tortillas with cooking spray. Cut into 2- x 1/2-inch strips. Place in a single layer on a rimmed baking sheet. Bake in preheated oven until browned, about 10 minutes.
Place tomato mixture in a blender. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening, and process until smooth, about 1 minute. Pour soup into bowls. Top with tortilla strips, sour cream, and cilantro.