Melt butter in a large skillet over medium heat. Add onions and cook, stirring occasionally, until soft and just beginning to brown, about 20 minutes. Remove from the heat and stir in chicken, pimentos, and cream cheese. Season with salt and set aside.
Heat 1/8 inch oil in a small skillet over medium heat. Use tongs to dip 1 tortilla into the hot oil just until it blisters and turns limp, about 3 seconds; do not fry until firm or crisp. Continue with remaining tortillas, stacking them on a plate between paper towels when heated.
Spoon about 1/3 cup filling down the center of each tortilla, then roll to enclose. Place enchiladas, seam-side down, in a 9x13-inch baking dish. Cover and refrigerate for up to 24 hours.
When ready to bake, preheat the oven to 375 degrees F (190 degrees C).
Remove enchiladas from the refrigerator. Spoon cream over top and sprinkle with Monterey Jack cheese.
Bake, covered, in the preheated oven for 15 minutes. Uncover and bake until hot and bubbly, about 15 minutes longer.