Cover the chiles with boiling hot water and let them soak for 10 minutes. Pace the chiles along with 1/2 cup of soaking liquid, onion, garlic, oregano, vinegar, sugar and salt in the blender. Purée until smooth.
In a saucepan set over medium heat, heat the oil. Once hot, pour the sauce into the oil; cover the saucepan, leaving it slightly open, and let the sauce season and thicken for 6 to 8 minutes, stirring here and there. Remove from the heat.
To make the Fish:
Baste the fish fillets with the adobo sauce; you may refrigerate them and let them marinate for up to 24 hours.
In a large skillet coated with oil and set over medium-high heat, cook the fish for about 3 to 4 minutes per side.
To make the Salsa:
Heat a grill pan, a grill or a nonstick skillet over medium heat until hot and lightly coat with safflower or corn oil. Cook the pineapple slices for about 4 minutes per side until they are slightly charred. Remove from the heat, once cool enough to handle cut into bite-size chunks. Place in a mixing bowl and toss with the cilantro, chile, lime juice, oil and salt to taste.
To assemble Tacos:
Place the warm tortillas, the pineapple salsa and the fish on the table, then assemble the tacos!