For the steaks: In a small bowl, combine the paprika, thyme, salt and a few grinds of black pepper. Sprinkle on both sides of the steaks and let rest at room temperature for up to 2 hours.
In a large pan with straight sides, add the olive oil over medium-high heat. Add the steaks and sear on both sides until a deep golden color is on the bottom, 4 to 5 minutes. Flip and cook about 4 minutes more. Remove to a plate and cover with aluminum foil.
For the gravy: Using kitchen twine, tie the thyme and rosemary together. In the same pan you seared the steaks in, add the herb bundle and the butter. Lower the heat to medium, melt the butter and then add the onions. Season with a pinch of salt and a grind or 2 of black pepper. Cook on medium heat until the onions are tender, about 5 minutes, and then add the garlic. When the garlic becomes fragrant, about 2 minutes, add the stone-ground mustard and beef stock. Stir to blend and then add the heavy cream. Taste and season with more salt, if needed, and raise the heat to a gentle simmer. Once the sauce has thickened to a gravy, lower the heat and stir in the sour cream. Add the reserved steaks back into the pan and bring to a simmer while spooning the sauce over the steaks. Cook this way until desired doneness; it should take only a minute more for medium-rare. Remove from the heat and allow to rest for 5 minutes before slicing. Place the slices on a serving plate and pour the gravy over the top.