In a small bowl, combine the sour cream and olive oil, and whisk together (it will look a little curdled, but don't worry). Add the remaining dressing ingredients and whisk to blend (now it should be smooth). Set aside.
In a large bowl, combine the olive oil, garlic, and cumin and mix well. Add the bread cubes and toss well to coat. Put the seasoned cubes on an ungreased baking sheet and bake in a 350 degree F. oven until crisp on the outside but still chewy inside, about 10 minutes.
In a large bowl, combine the romaine lettuce, avocados, tomatoes, and onion. Stir the dressing well, add just enough to moisten the ingredients (there will be some dressing left over), and toss to coat. Sprinkle with cheese and croutons, and serve. Serves 6.
Each serving contains 603 calories, 9g protein, 49 fat, 36g carb, 697mg sodium, 32mg cholesterol.