Melt butter in a saucepan over medium-high heat. Add flour and whisk until smooth, at least 2 minutes. Slowly add milk, whisking continuously until mixture is smooth, about 5 minutes. Stir half-and-half into the saucepan; season with salt and pepper. Add liquid smoke. Cook and stir until gravy is smooth and slightly thickened, 2 to 3 minutes more.