Remove the shells and tails from the shrimp and reserve. Rinse the shrimp and pat dry. Cut each one into 3 or 4 bite-size pieces.
In a medium saucepan, combine the shrimp shells and tails, parsley, 1 of the garlic cloves, bay leaves and ½ teaspoon of the salt. Cover with water, place over medium-high heat and bring to a boil. Reduce heat to medium and simmer 20 minutes. Strain the broth into a large measuring cup or heat-proof bowl.
Combine the tomatoes, remaining garlic, and chile(s) in a medium saucepan. Cover with water, and bring to a simmer over medium-high heat. Reduce the heat to medium and simmer uncovered until the tomatoes are thoroughly soft, about 10 minutes.
Using a slotted spoon, transfer the tomatoes and garlic clove to a blender. Add the 4 coarsely chopped scallions; if you have simmered 2 chiles, begin by adding only 1 of them to the blender, then once you taste the finished puree you can decide if you want to add the other. Add ½ teaspoon salt, the grated nutmeg, and 1 cup of the strained shrimp shell broth. Purée until completely smooth, taste and add the other chile if you would like more chile presence and heat (the sauce will become milder as it cooks and other ingredients are added).
Rinse and dry the saucepan, add a tablespoon of the oil and heat over medium heat until hot but not smoking. Add the pureed tomato sauce, being careful to avoid sputters, and cover partially with a lid. Simmer, stirring occasionally, until the sauce thickens, seasons and deepens in color to a much darker red, about 10 minutes. Uncover, reduce heat to medium-low and stir in the cream. Keep at a steady low simmer for 8 to 10 more minutes, or until the sauce is thick, creamy, and coats the back of a wooden spoon. Taste and adjust salt. Remove from the heat and keep warm.
Prepare the tortillas for enchiladas by either heating them in a comal or passing them through hot oil.
To cook the shrimp, work in batches so that they will sear and not steam. Heat 1 tablespoon of the oil and 1 tablespoon of the butter in a large skillet over high heat until hot, but not smoking, and the butter is bubbling. Add half the shrimp and season with salt. Cook for just a couple of minutes, stirring and flipping a few times, until just cooked through and lightly browned. Remove with a slotted spoon, add the remaining tablespoon of oil and butter to the skillet, and once the butter is foaming, cook the remaining shrimp.
Reheat the sauce if necessary. One by one, sauce and fill the tortillas. Glide each prepared tortilla through the sauce and place on a plate. Spoon about 3 tablespoons of the seared shrimp down the middle and fold in a half like a quesadilla. Place on a platter and continue with remaining the tortillas, overlapping the half moon-shaped enchiladas slightly. Once all of the enchiladas are filled, spoon the remaining tomato sauce on top. They should be sauced generously. Garnish with the sliced scallions, slices of avocado and crumbled cheese.