Preheat the oven to 400 degrees F.
Place the tomatillos, jalapeno, and red onions on a baking sheet. Drizzle with olive oil and sprinkle with a pinch of salt and a grind of black pepper. Place in the oven and roast until the vegetables begin to brown and slightly blacken, 25 to 30 minutes. Remove all contents from the baking sheet to a food processor and allow to cool slightly before blending. Add the cumin, crema, lime juice, and season with salt and pepper, to taste. Puree until smooth. Refrigerate at least 1 hour before serving. Immediately before serving, toss the dressing with the lettuce, tomatoes, and radishes.