Pre-heat your oven to 375F.
Heat an oven proof skillet over medium heat. Season the steaks with salt and pepper. Add1 Tbsp butter and 1 Tbsp olive oil to the pan until the butter stops sizzling. Watch out - you don't want the butter to burn.
Add the steaks and pan fry for about 90 seconds per side then place in pre-heated oven for about 3-4 minutes for rare to medium rare. I can't predict this for you. Steaks are different thicknesses. Use your instant read thermometer to be certain - 125F is the magic number for beef.
Remove pan from oven, put a cloth on the handle to remind you it's screaming hot, and remove steaks. Place pan over medium heat. Add white wine, scraping up any browned bits and reduce to a syrupy glaze. This is called deglazing. Add veal stock or demi-glace and simmer briefly. Whisk in the mustard.
Off heat, whisk in the remaining 2 Tbsp butter one tablespoon at a time. Taste sauce and adjust seasoning if necessary.