Heat oven to 450°F. In small bowl, combine 1/2 teaspoon of the salt, the garlic powder, paprika and pepper; mix well. Rub mixture on all surfaces of beef tenderloin. Place beef in small shallow roasting pan; tuck thin end under.
Bake at 450°F. for 20 to 25 minutes or until meat thermometer inserted in center registers 140°F. for rare. Cool 30 minutes or until completely cooled. If desired, wrap tightly; refrigerate until serving time.
Meanwhile, in large skillet, heat butter and oil over medium heat until butter melts. Add brown sugar and onions; stir to coat. Cook over medium heat for 10 minutes or until onions begin to soften, stirring occasionally. Add wine. Reduce heat to medium-low; cover and cook 10 to 15 minutes or until onions are very tender. Cover; refrigerate until serving time.
To serve, in small bowl, combine sour cream, horseradish sauce and remaining 1/2 teaspoon salt; mix well. Heat focaccia as directed on package; cut in half horizontally. Spread both cut sides with sour cream mixture. Arrange spinach leaves on bottom half of focaccia.
Slice beef very thin; layer over spinach. Top with onions and top half of focaccia; press down. Insert toothpicks through all layers at 1 1/4 to 1 1/2-inch intervals. Cut between toothpicks to form tiny sandwich squares.