Preheat oven to 375°F. Lightly coat an 11-by-7-inch baking dish with cooking spray. Cut about ¼ inch off tops of 6 peppers; remove and discard ribs and seeds. Coarsely chop the pepper tops, discarding stems; set aside the chopped peppers (about ½ cup). Place the whole peppers, cut-sides down, in the prepared baking dish; bake until just slightly softened, about 15 minutes. Let stand at room temperature until cool enough to handle, about 10 minutes. Leave the oven on.

While the peppers cool, heat 1 tablespoon oil in a large Dutch oven over medium heat. Reserve 2 tablespoons scallions. Add the remaining scallions (about 1 cup plus 6 tablespoons), the reserved chopped peppers and the chopped sweet potato to the pot; cover and cook, stirring frequently and adjusting the heat as needed to prevent overbrowning, until softened, 6 to 8 minutes. Add 3 cups baby spinach; cook, uncovered and stirring constantly, until wilted, 1 to 2 minutes. Stir rinsed beans, undrained tomatoes, prepared rice, 2 tablespoons cilantro and 1 teaspoon each salt, garlic powder, cumin and paprika until well combined. Stir in 1 cup cheese; remove from heat.

Flip the peppers cut-sides up in the baking dish; divide the sweet potato mixture evenly among the peppers, mounding and pressing lightly if needed. Top with the remaining 1 cup cheese. Bake until the peppers are tender, the filling is hot and the cheese is melted, 20 to 25 minutes. Sprinkle with the remaining 2 tablespoons scallions and ½ tablespoon cilantro.