Preheat the oven to 325 F. Rinse the turkey and pat dry with paper towels. Place turkey on a rimmed baking sheet or roasting pan.
Combine 1-1/4 teaspoons kosher salt, two tablespoons oil, minced garlic, four thyme sprigs, and three rosemary sprigs in a small bowl; rub all over turkey.
Turn the turkey breast side up on the prepared baking sheet so drumsticks are pointed toward the opening of the oven.
Pour apple cider into the cavity, and tie legs together with kitchen twine.
Bake turkey until a thermometer inserted in the thickest part of the thigh (not touching bone) registers 165 degrees F and breast temperature reach 155 degrees F, 1 hour 45 minutes to 2 hours 15 minutes.
Baste every 30 minutes with pan juices. If desired, place red onions and mushrooms around the turkey during the last 30 minutes of bake time.
Remove turkey from oven; transfer to a platter and let stand, loosely covered with foil, at least 30 minutes before carving (internal temperature will rise 5 degrees F).
Discard herb sprigs, and drain fat from the roasting pan. Place the roasting pan on the stove; heat pan juices to simmer over medium heat.
Add remaining 3/4 teaspoon salt and chopped thyme and rosemary; cook until reduced by half.
Remove from heat, whisk in one tablespoon oil and Dijon mustard; season with salt and pepper, if desired. Carve turkey, and serve with gravy.