Remove 8 leaves from cabbage. Blanch in a large saucepan of boiling salted water (1-2 minutes), drain and refresh, drain, pat dry with absorbent paper, set aside. Cut cabbage heart in half, thinly slice one half (reserve remaining half for another use), set aside.
Sauté speck in a large frying pan over medium-high heat until golden (2-3 minutes). Remove speck, drain on absorbent paper, set aside. Heat half the butter in frying pan over medium heat, add sliced cabbage, sauté until tender (15-20 minutes). Add blue cheese, walnuts and speck, season to taste, mix until combined. Divide cabbage mixture among blanched cabbage leaves, fold ends over, then roll tightly to form cylinders.
Heat remaining butter in a large frying pan, lightly dust cabbage leaves in flour, shake off excess, and cook, turning occasionally, until golden (3-4 minutes). Serve warm.