In a large bowl, add meatloaf ingredients and mix well. Lightly oil a 9 x 5-inch loaf pan and add meatloaf ingredients. Shape meat mixture into a loaf. Bake uncovered 45 minutes, or until the center has lost its pink color.
During the last 10-15 minutes of cooking the meatloaf, make the gravy. In a large skillet on medium-low heat, add the butter. When melted, add mushrooms and cook until softened, about 8-10 minutes.
Add flour, cook, and stir until lightly browned. Slowly add broth and balsamic vinegar, whisking to combine. Bring to a boil over medium-high heat. Once boiling, reduce to medium-low and continue cooking until gravy has thickened, 5-10 minutes. Remove from heat and add salt and pepper.
Remove meatloaf from pan, slice, and add gravy over the sliced loaf.