Preheat the oven to 325 degrees F (165 degrees C).
Melt butter in a saucepan over medium heat. Add Dijon mustard and garlic; whisk well.
Spray a baking dish with cooking spray; add tilapia. Brush fillet with most of the butter mixture. Top with crumbled crackers. Brush more sauce on top. Pour the dregs of the sauce over the fish if desired.
Bake in the preheated oven until fish flakes easily with a fork, about 25 minutes.
