Preheat the oven to 350°, and prepare a 9x13" greased casserole dish.
To make the sauce, bring 2 tablespoons of oil to temperature in a large skillet over medium heat. Add the jalapenos and onions and cook, stirring often, for about 10 minutes. Add the garlic and cook 1 minute more.
Transfer the peppers, onion, and garlic to a food processor fitted with a blade attachment or a blender. Add the cilantro, secure the lid, and remove the feed tube. Cover the opening with a dish towel and blend until smooth and pureed. Set aside and return the pan to medium-high heat.
Add the remaining 2 tablespoons of oil to the pan, allow to heat through, and sprinkle with flour. Whisk the flour into the oil for about 2 minutes.
Begin adding the half and half in one big splash at a time, whisking well after each addition. Stir in the Kosher salt and smoked paprika, and allow the mixture to come to a boil. Reduce the heat to maintain a simmer, and allow the mixture to simmer for about 5 minutes, just until slightly thickened, stirring often.
Stir in the pureed peppers and onions and remove from the heat. Set aside while you cook the shrimp.
To cook the shrimp, ready a large pot of water over high heat. Bring to a boil and add the shrimp. Allow the shrimp to boil for about 2 minutes. As soon as the shrimp are cooked through, quickly use a slotted spoon to remove the shrimp and transfer to a work surface.
Once the shrimp are cool enough to handle, cut them into small, bite-sized pieces.
Transfer the chopped shrimp to a small mixing bowl, along with about ¾ cup of the sauce and 1 ½ cup of shredded cheese. Mix with a fork to combine.
To warm the tortillas for rolling, stack 4 tortillas in the center of a damp paper towel, and microwave for 1 minute. Repeat until all of the tortillas are warm and pliable.
Add a ladleful of sauce to the bottom of the casserole dish and begin rolling your enchiladas.
Add about 2-3 tablespoons worth of filling to each tortilla and roll tightly. Place seam-side down across the casserole dish. Pour over the remaining sauce, and top with the remaining cheese.
Bake 25-30 minutes, until the sauce is bubbly and the cheese is melted. Serve and enjoy.