To make a flat 3/4-inch-thick piece of beef, butterfly tenderloin by cutting, from top to bottom, through center to within 3/4-inch of bottom; open like a book and press to flatten. Cut horizontally, starting at the center of each half, to within 3/4-inch of each side. Open and flatten tenderloin slightly with a meat pounder or side of a meat cleaver or Chinese cleaver so that whole piece is about the same thickness.
In a large skillet over medium heat, saute diced bacon until golden brown. When fat is rendered and bacon is crisp, add spinach and cook until wilted (about 6 minutes). Place spinach-bacon mixture over tenderloin. Roll up tenderloin, jelly-roll fashion. Tie with kitchen twine in 4 places to fasten tightly.
In a small bowl mix together mustard and brandy. Rub over tenderloin. Sprinkle with the Omaha Steak All Natural Seasoning. At this point, meat can be refrigerated for about 8 hours, if desired; bring meat to room temperature 1 hour before cooking.
Preheat oven to 425° F. Return skillet to medium heat, add olive oil, and place tenderloin, seam side down, in skillet; brown on all sides (5 minutes per side). Place tenderloin in oven; roast until medium-rare (about 40 minutes, or until internal temperature is 125° F on an instant-read thermometer). Let tenderloin rest 12 to 15 minutes, loosely covered with aluminum foil, before slicing. Slice into 6-8 pieces across the grain of the meat, and serve immediately.