One-Pot Broccoli-Cheddar Pasta

Ingredients: 8 ounces pasta (such as penne, fusilli, or your favorite shape) 2 cups broccoli florets 2 tablespoons butter 2 cloves garlic, minced 1/4 cup all-purpose flour 2 cups milk 1/2 teaspoon salt 1/4 teaspoon black pepper 1/4 teaspoon ground mustard (optional) 2 cups shredded sharp cheddar cheese