Heat 2 tablespoons oil in a pot over medium heat. Add the onion and saute until golden. Add the garlic and saute for a minute. Increase heat to medium-high; add the ground beef and brown until no longer pink, stirring occasionally. Reduce heat to medium, add the diced bell pepper, crushed tomato, turmeric, oregano, salt and pepper. Stir well and saute for 1-2 minutes. Add 1/4 cup water, cover the pot and cook over medium heat for 20 minutes. Remove from heat, add Parmesan cheese and cilantro. Taste, adjust seasonings and set aside.
Make the White Sauce or Bechemal Sauce and set aside.
Cook the shells according to package instructions in salted water until al dente (don't overcook shells or they'll tear apart). Drain and set aside to cool.
Preheat oven to 360 degrees F. Spray two 11x8-inch baking dishes with cooking spray. Spread a little of white sauce over bottom of the baking dishes.
Fill each shell with beef mixture. Arrange them in a single layer in the baking dishes.
Spoon remaining white sauce over the shells.
Cover the baking dish with foil and bake for 35 minutes.
Remove from the oven and sprinkle mozzarella cheese over the shells. Bake uncovered for about 10-15 minutes or until cheese is melted and lightly golden. Garnish with cilantro and serve immediately.