Place the rice in a colander or a fine-mesh strainer and rinse under cold water until the water runs clear. Combine the rice, measured water, and 3/4 teaspoon of the salt in a medium saucepan and bring to a boil over high heat. Cover the pan and reduce the heat to low; cook until the water is completely absorbed, about 10 minutes (the rice will be slightly undercooked). Remove the lid and set the pan aside to let the rice cool. Meanwhile, prepare the peppers. Use a paring knife to cut a wide circle around each stem (like when carving a jack-o’-lantern), so you end up with a cap that can be replaced once you’ve stuffed the peppers—be careful not to puncture or rip the peppers. Remove and discard any seeds and membranes from the cap and from the interior; set the peppers aside. Place the beans in a large bowl. Using a potato masher or the back of a fork, lightly mash them (some whole beans should remain). Add the tomatoes, scallions, Cotija, sour cream, cilantro, cumin, measured black pepper, and remaining tablespoon of salt and mix until evenly combined. Gently mix in the cooled rice. Taste the mixture and, if necessary, season with more salt and pepper. (Note that the filling should be quite salty to compensate for there being no salt on the peppers.) Divide the rice mixture into 6 equal portions. Stuff each pepper with the filling, replace the caps, and press each cap into the filling. Pierce 2 toothpicks through each cap and out the sides of each pepper to secure them while in the oven; set aside until ready to cook. (If you’re stuffing the peppers more than 30 minutes before grilling, cover and refrigerate them for up to 4 hours. Let the peppers sit at room temperature at least 30 minutes before grilling so that they cook faster and more evenly.) Heat the oven to 425°F and arrange a rack in the middle. Place the stuffed peppers on their sides on a baking sheet (make sure the peppers aren’t touching each other). Bake until the peppers are softened and a little blistered and the filling is heated through, 25 to 30 minutes. Remove the toothpicks. If serving as a side dish, slice the peppers in half lengthwise.