1.Combine half of the mustard and half of the horseradish in small bowl; spread over steaks. Refrigerate, covered, until required.
2.For crostini, preheat oven to moderately slow. Discard ends of bread; cut into 1cm slices. Place slices, in single layer, on oven tray; spray with oil. Toast, both sides, in moderately slow oven.
3.Heat oil in large frying pan; cook steaks, uncovered, until cooked as desired. Cover; stand 5 minutes then slice thinly.
4.Combine remaining mustard and remaining horseradish with sour cream in small bowl. Top crostini with spinach, steak slices and sour cream mixture.