Heat a heavy skillet over medium-high heat. Add 1 tbs olive oil.
Brown meat on all sides; remove from pan and set aside.
In the same skillet add 1 tbs olive oil and the butter over medium-high heat. Add the mushrooms and sear until they begin to brown. Lower the heat to medium-low then add the onion and sauce until translucent.
Add the remaining sauce ingredients stir well to combine. Add the sirloin back to the pan and heat through.
Sour cream is a decent substitute for creme fraiche. See link in post for a copycat recipe.