Pat the steaks dry with paper towels and season generously with salt and pepper
Heat a cast iron skillet over medium-high heat until very hot, about 2 minutes
Add 1 tablespoon butter to the skillet and swirl to coat
Working in batches if necessary, sear the steaks for 4 to 5 minutes on each side for medium-rare, or until desired doneness is reached
Transfer the steaks to a warm plate and tent loosely with foil to keep warm
Reduce heat to medium and add the remaining tablespoon of butter to the skillet
Add the minced shallots and sauté for 2 to 3 minutes until softened and fragrant
Add the red wine and scrape up any browned bits from the bottom of the skillet using a wooden spoon
Add the beef stock, thyme sprigs, and bay leaf, then simmer for 8 to 10 minutes until the sauce has reduced by half and thickened slightly
Stir in the Dijon mustard and lemon juice
Taste and adjust seasonings as needed
Remove from heat and strain the sauce through a fine-mesh strainer if desired, discarding the solids
Spoon the sauce over the steaks and serve immediately
Serve immediately with potatoes on the side