1 tablespoon vegetable oil
10 ounces beef sirloin (or fillet cut into 1/3” wide strips)
1 tablespoon butter
1 onions (small, diced)
9 ounces button mushrooms (halved, quarter if they are large)
1/4 cups dry white wine
1 tablespoon tomato paste
1 teaspoon whole grain mustard
3/4 cups beef stock
2 teaspoons flour
1/4 cups sour cream