Preheat the oven to 400℉. Spray a 9x13-inch baking dish with nonstick spray.
Heat a large skillet over medium heat. Add the ground beef and sprinkle with ¾ teaspoon of the salt and pepper. Cook until no longer pink, about 10 minutes, breaking up any clumps with a wooden spoon. Add the garlic and red pepper flakes and cook for another minute.
In a large bowl, combine the ricotta, spinach and Parmesan. Add the cooked beef and stir to combine.
Cut each bell pepper in half lengthwise and remove the seeds and ribs, keeping the stem as intact as possible. Set the pepper halves into the prepared baking dish. Spoon the beef and ricotta mixture into each half, mounding it on top.
In a medium bowl, whisk the tomato paste with the water until smooth. Whisk in the garlic powder and remaining ½ teaspoon of salt. Spoon the tomato sauce over the peppers. Sprinkle with the mozzarella.
Bake for 30 minutes, then turn the oven to broil and broil the peppers 6 inches from the heat for 1 to 3 minutes, until lightly browned. Remove from the oven and sprinkle with the chopped basil.
Store leftovers in the refrigerator for up to 4 days.