Preheat the oven to 400°F. Lightly oil a shallow 2-quart baking dish or spray with nonstick cooking spray.
In a large bowl, combine the cooked bulgur, tomatoes, black beans, 1 1/2 cups cheese, and half the scallions. In a small bowl whisk together the sour cream, heavy cream, chicken broth, pureed chipotles, and salt and pepper. Add the sour cream mixture to the quinoa mixture and mix well.
Place half of the bulgur mixture in the prepared pan, layer over half the sliced sweet potatoes, barely overlapping the edges of the slices, and repeat using the rest of the bulgur mixture and the rest of the sweet potatoes. Sprinkle the remaining cup of cheese over the casserole, cover with foil and put in the oven. Bake for 20 minutes, then remove the foil, and continue to bake for 35 to 40 more minutes, until browned and bubbly, and a knife inserted in the casserole slides in easily thorough the potatoes.
Remove for the oven, let sit for 5 minutes, sprinkle the rest of the scallions on top, and serve hot. Serve with lime wedges and pass the additional sour cream, if desired.