Place the chicken tenderloins in a large bowl and pour in the buttermilk to fully submerge; cover and refrigerate for at least 2 hours or overnight.
In a shallow bowl or plate, whisk together the flour, salt, black pepper, and garlic salt until evenly combined.
Stir about 1/2 cup of the reserved buttermilk into the flour mixture until it forms a shaggy, slightly wet batter; discard any remaining soaking buttermilk.
One at a time, dredge each chicken tenderloin in the batter to coat completely, then place on a wire rack and let rest a few minutes so the batter adheres.
Heat vegetable oil in a deep fryer or large heavy-bottomed pot to 350°F (175°C). Use enough oil so strips can float while frying.
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; the internal temperature should reach 165°F (74°C).
Remove the strips with tongs and drain on a wire rack set over a rimmed baking sheet; repeat with remaining strips.
Serve the chicken strips hot with honey mustard or your preferred dipping sauce.
