Meanwhile, remove 1 tsp. zest and squeeze 1 Tbsp. juice from lemon. In a 10-inch skillet heat 1 Tbsp. of the oil over medium. Add onion; cook 4 minutes, stirring occasionally. Add linguine, the reserved pasta cooking water and 1/4 tsp. of the pepper. Cook 4 minutes or just until pasta is tender and most of the water is absorbed, stirring frequently. Add lemon juice, 1/4 cup of the cheese, the broth and butter. Cook and stir 2 minutes more.