Heat a large skillet over medium heat. Add the beef and sprinkle with ¾ teaspoon of the salt and the pepper. Cook until no longer pink, about 10 minutes, breaking up any clumps with the back of a wooden spoon. Add the garlic and red pepper flakes and cook another minute.
Squeeze the spinach to remove the excess moisture. In a large bowl, combine the ricotta, spinach, and parmesan. Add the cooked beef and stir to combine.
Cut each bell pepper in half and remove the seeds and ribs. Set the pepper halves in a 9x13 glass or ceramic baking dish. Spoon the beef/ricotta mixture into each half, mounding the tops.
In a medium bowl, whisk together the tomato paste, water, garlic, and remaining half teaspoon of salt. Spoon over the tops of the peppers and sprinkle with the mozzarella.
Bake 30 minutes, then turn the oven to broil and and broil the pepper 6 inches from the heat for 1 to 3 minutes, until lightly browned. Remove from the oven and sprinkle with chopped basil.