Heat 2 Tbsp olive oil in a large sauté pan over medium heat. Add the beef in a single layer, cooking on each side until seared golden brown. You may need to work in batches to not overcrowd the pan, cooking each batch about 2 minutes per side.
Remove from the pan and season with salt and pepper. Cover and keep warm. Reserve pan and drippings to make the sauce (below).
In another small skillet, preheat the remaining 1 Tbsp of olive oil to medium-high heat. Add the leeks and garlic and cook for 1 minute. Stir in the cabbage and continue to cook just until slightly soft. Gently combine into the potatoes.
In the same pan used for the steaks, heat 1 Tbsp of olive oil over medium-high heat. Add shallots and sauté for about 4 minutes.
Reduce heat, whisk in the flour and stir for 20 seconds. Gradually pour in the broth, whisking as you add a bit at a time until the sauce thickens slightly.
Stir in the mustard and the thyme. Remove from the heat and stir in 1 Tbsp butter. Adjust seasonings if needed.
Plate potatoes next to two steak medallions and drizzle steaks with sauce.