Mix sirloin, salt and pepper well. Form 4 patties about 4 inches wide.
Warm oil in a large skillet over mediumhigh heat. Cook patties until browned, 5 minutes per side. Transfer to a plate; cover with foil.
Pour off all but 1 Tbsp. fat from skillet. Add onion and cook, stirring, until softened, 2 minutes. Add mushrooms; continue cooking, stirring, until mushrooms have given off their liquid and are soft, 10 minutes. Stir in cream and cook until slightly thickened, 2 minutes. Stir in parsley; return patties to skillet. Turn a few times over low heat until patties are coated in sauce and warmed through, 3 minutes. Spoon mushrooms on top and serve.