Heat 3 Tbl butter with 1 Tbl olive oil in a large stockpot. Add mushroom and cook until mushrooms are brown and liquid has evaporated. Transfer mushrooms to a bowl.
Heat 1 Tbl butter with 1 Tbl olive oil in a medium saucepan. Add the onion and cook until transparent. Add cremini mushrooms and cook for 2 minutes. Add the stock and cook 20 minutes. Season to taste with salt & pepper. Strain the stock through a strainer into the stockpot that you sauteed the mushrooms in and discard the onions and mushrooms.
Add the sauteed mushrooms and cook for 15 minutes. Season to taste with salt & pepper.
Slice the sirloin steak into thin slices and cut those into bite size pieces. Saute and then add to soup mixture.