Ideal for cozy family suppers, this leftover-friendly, quick-to-prepare entrée would be delicious served with a jìcama-orange-radish salad with lime vinaigrette.
Heat the oven to 400°F. Poke each yam all over with a knife, place in the microwave, and cook on high until very tender, about 13 minutes (this will depend on the strength of your microwave). When cool enough to handle, cut them open and scoop out the flesh; transfer 2 1/2 cups to a medium-sized bowl (reserve any remainder for another use). Mash with a potato masher. While still hot, add the cheese, milk, butter, 3/4 teaspoon salt, and 5 grinds pepper, and stir to mix well. Set aside.
Meanwhile, heat the oil in a 10-inch, heavy sauté pan (with high sides) over medium-high heat. When hot, add the onions, peppers, celery, and garlic, and sauté until the vegetables are softened, about 6 minutes. Add the beans, tomato paste, vinegar, honey, 1/4 plus 1/8 teaspoon salt, 1/4 teaspoon cumin, 1/8 teaspoon chili powder, and 5 grinds pepper. Cook, stirring to combine the ingredients well, for another 5 minutes. Set aside off the heat.
Grease a 2-quart rectangular glass baking dish with nonstick cooking spray. Add the bean mixture and spread evenly. Top with the yam mixture and spread evenly. Sprinkle with the remaining 1/8 teaspoon chili powder and 1/8 teaspoon cumin. Bake until hot and the edges of the yam mixture turn golden brown, about 35 minutes. Serve with sour cream and cilantro, if using.