2 tablespoons vegetable oil
1 pound beef sirloin, cut into 2x1x1/8-inch strips
Kosher salt and freshly ground black pepper
6 tablespoons unsalted butter
12 ounces button mushrooms, quartered
1 medium yellow onion, thinly sliced
1 tablespoon tomato paste
2 tablespoons all-purpose flour
2 cups beef broth, homemade or low-sodium canned
1/4 cup sour cream
2 teaspoons Dijon mustard
2 teaspoons freshly squeezed lemon juice
1 tablespoon chopped flat-leaf parsley leaves
For noodles:
Kosher salt
One 12-ounce package wide egg noodles
2 tablespoons unsalted butter
Freshly ground black pepper