Preheat the oven to 375 degrees F.
Heat the butter in a heavy medium saute pan over high heat. Add the garlic and saute 10 seconds. Add the shrimp and saute until cooked through, about 3 minutes. Season with salt and pepper and let cool slightly.
In a medium saucepan, heat the Guajillo Salsa over medium heat until warm.
In a separate medium saute pan over medium-high heat, pour enough vegetable oil to come 1-inch up the sides of the pan. Fry the tortillas until slightly golden but still pliable. Then transfer to the pan with the salsa, turning the tortillas in the salsa until fully coated.
Transfer the salsa-coated tortillas to a baking sheet (it's kind of messy so doing it on a baking sheet keeps it clean). Add a couple tablespoons of sauteed shrimp and a sprinkle of Oaxaca cheese. Roll up like a cigar and place in an 11-by-7-inch glass baking dish. Repeat with 7 or 8 tortillas, fitting snuggly into the dish.
Pour the remaining sauce over the prepared enchiladas (There should be some sauce left in the saucepan to top the enchiladas. If necessary, whisk in 2 tablespoons of water or chicken broth to increase the yield.) Top with the remaining Oaxaca cheese.
Bake in the oven until the cheese melts, about 20 minutes. Drizzle with crema and serve.
In a large saucepan over medium-high heat, place 4 cups water, the guajillo chilies, garlic cloves and white onion and bring to a boil. Reduce the heat to simmer until the chiles are soft and tender, about 25 minutes. Transfer the boiled vegetables, the tomato and 2 cups of the boiling liquid to a large blender and set aside to cool before blending. Process until smooth. Season with 1 teaspoon salt and a pinch of black pepper.
In a medium, heavy saucepan, heat the vegetable oil over medium heat. Strain the pureed guajillo salsa into the pan. Add the bay leaf and simmer until thickened, about 10 minutes. Taste for seasoning, adding additional salt and pepper as desired.
Reserve for enchiladas and pork.
For ease of preparation, you can use 2 store-bought, canned whole tomatoes, seeded, in place of 1 fresh, boiled tomato.