Roasting your own beef helps make this sandwich staple extra-special.
Preheat oven to 425 degrees. Heat a large ovenproof skillet over medium-high heat until warm. Add oil and butter; heat until oil is hot and butter has melted.
Season beef with salt and pepper; place in skillet. Cook until beef is well browned on all sides, about 5 minutes.
Transfer skillet to oven. Bake until an instant-read thermometer registers 130 degrees for medium-rare, 18 to 22 minutes. Transfer beef to a cutting board; let rest at least 20 minutes before slicing.
Slice beef very thinly crosswise. Peel and pit avocados; cut into 1/2-inch-thick slices. Arrange tomatoes, lettuce, radicchio, avocados, and beef over half the bread. Spread horseradish cream onto each remaining bread slice, and place on top of sandwich, spread side down.
To make slicing easier and more precise, make the beef a day ahead and refrigerate it overnight.