Season both sides of each steak with freshly ground pepper and salt. Heat oil in a heavy non-stick pan over medium-high heat. Sear steaks for 1 minute on each side until browned, then transfer to a plate.
In the same pan, add the sliced red onion, garlic, and a dash of Tabasco. Pour in 125 ml red wine and 125 ml vegetable stock. Cover and cook over high heat for 2-3 minutes until the onion is soft and the liquid has reduced to a syrup-like consistency. Stir in any meat juices from the plate.
Return steaks to the pan and cook uncovered for 1-2 minutes, turning once. For medium-rare to medium doneness, cook until the internal temperature reaches 63-71°C (145-160°F). Transfer to a clean plate.
Pour the remaining wine and stock into the pan. Simmer over medium heat for 3-4 minutes until the liquid has reduced by half. Whisk in the Dijon mustard and cook for 2 minutes until the sauce thickens.
Return the steaks to the pan and turn to coat with sauce, cooking for 1 minute until heated through. Transfer steaks to serving plates and spoon the sauce over them.