Place the chicken tenderloins in a large bowl and pour the buttermilk over them. Cover and refrigerate for at least 2 hours or preferably overnight.
In a shallow bowl or pie plate, whisk together the flour, salt, black pepper, and garlic salt until evenly combined.
Add about 1/2 cup of the used buttermilk to the flour mixture and stir until a shaggy batter forms; discard the remaining buttermilk from the soaking bowl.
Working with a few pieces at a time, coat each tenderloin in the batter, shaking off excess, and place on a wire rack; let rest a few minutes so the batter adheres.
Heat vegetable oil in a deep fryer or heavy-bottomed pot to 350°F (about 175°C) and maintain temperature.
Carefully add 3–4 breaded strips to the hot oil without crowding and fry until golden brown and cooked through, about 3–5 minutes depending on thickness.
Remove the strips with tongs and drain on a wire rack set over a rimmed baking sheet. Repeat with remaining chicken.
Serve the chicken strips hot with honey mustard or your preferred dipping sauce.