Preheat oven to 325°F.
Pat tenderloins dry with paper towels. Rub mustard evenly over both tenderloins.
Stir together brown sugar, thyme, garlic powder, pepper, sage, and 1 ½ teaspoons of the salt in a small bowl. Sprinkle mixture evenly over both sides of the tenderloins, pressing lightly to adhere. Set aside.
Heat oil in a large ovenproof skillet over medium-high until oil shimmers. Add tenderloins, and cook until lightly browned, about 2 minutes per side.
Transfer skillet to preheated oven, and cook until a thermometer inserted into thickest portion of tenderloins reaches 160°F, 15 to 20 minutes. Transfer turkey to a cutting board. Let rest 10 minutes. Temperature will continue to rise to 165°F. Do not wipe skillet clean.
While turkey rests, melt butter in the same skillet over medium. Whisk in flour, and cook, whisking constantly, until mixture is light brown and smells nutty, 1 to 2 minutes.
Slowly whisk in stock. Whisk in remaining ½ teaspoon salt. Bring to a simmer, whisking often, and cook until thickened to a gravy consistency, 3 to 4 minutes.
Remove from heat, and strain gravy through a fine mesh strainer. Slice turkey, and serve with gravy.
