Heat a large, deep skillet over medium high heat. Add 2 T olive oil.
Season tenderloin with salt and pepper and add to skillet. Brown pieces, remove from skillet and set aside.
Add onions and mushrooms to skillet with 2 T butter. Cook until the onions are starting to caramelize and the mushrooms are golden brown.
Add remaining 2 T butter to the skillet. After it has melted, sprinkle the flour over and whisk in. Cook for 1-2 minutes to cook off the raw flour taste. Add in sherry, cook until reduced by half, then whisk in stock.
Simmer until stock has reduced and mixture has thickened slightly. Add in mustard and season to taste with salt and pepper.
Remove from heat and stir in greek yogurt and sour cream.
Serve over hot buttered rice.