Salisbury Steak with Mushroom Gravy

Name: Salisbury Steak with Mushroom Gravy
Prep time: 10 min
Cook time: 35 min
Total time: 45 min
Cuisine: American
Ingredients: 1 1/2 tsp oil; 3/4 cup onions (minced); 1 lbs 93% lean ground beef; 1 lb 93% lean ground turkey; 1/2 cup unseasoned breadcrumbs (or gluten free crumbs); 1 large egg; 1 large egg white; 2 cups beef broth; 1/4 teaspoon kosher salt; 1/8 tsp ground black pepper (to taste); 8 oz sliced mushrooms; 2 tbsp all-purpose flour (or gluten-free flour blend); 2 tbsp tomato paste; 1 tsp red wine vinegar; 2 tsp Worcestershire sauce (to taste); 1/2 tsp mustard powder; 1/4 cup water; chopped parsley for garnish (optional)