Place the reserved bacon grease, butter, and minced garlic in a saucepan over medium-high heat; stir until butter is melted. Whisk flour into butter mixture, and continue whisking until flour starts to turn golden brown, about 1 minute. Slowly add milk, heavy cream, salt, and pepper to the saucepan, whisking until there are no lumps. Bring to a boil, stirring frequently. Reduce heat to medium.