Heat the oven to 400°F (200°C). Cut the peppers in half lengthwise and carefully remove the core, seeds, and ribs with a paring knife or spoon. Arrange the peppers, cut-side up, in a 9-by-13-inch baking dish or other rimmed dish in which they fit snugly. Set aside.
In a large (12-inch) skillet, heat the olive oil over medium heat. Add the celery and cook, stirring occasionally, until tender, 3 to 4 minutes. Crumble in the sausage and cook, breaking up the meat with a spoon, until no longer pink, for 5 to 6 minutes. Add the onion and garlic powders and the oregano. Stir for 1 minute.
Add the tomato sauce, 1½ teaspoons salt and ½ teaspoon pepper, and bring to a boil. Remove from the heat and stir in the frozen cauliflower rice and ¼ cup of the Parmesan cheese. Stir for 1 minute.
In a small bowl, combine the remaining Parmesan cheese with the shredded provolone. Set aside.
Place about ⅔ cup of the sausage mixture into each pepper half, dividing any extra evenly among the halves. Pour ¼ cup water into the bottom of the baking dish, wrap it tightly with foil, and bake until a paring knife easily pierces a pepper, 30 to 35 minutes. Remove the foil, spoon any accumulated juices onto the peppers, then sprinkle the cheese mix evenly onto the peppers. Bake until the cheese is melted and beginning to brown, 10 to 15 minutes. Let the peppers cool for 5 minutes, then serve.