Preheat oven to 200C. Combine stock, lentils, bay leaf and 300ml water in a saucepan, bring to the boil over medium-high heat, skim scum, reduce heat to medium, simmer until lentils are just tender and most stock is absorbed (15-20 minutes), season to taste, keep warm.
Meanwhile, for red wine cabbage, combine oil, onion and garlic in a large frying pan and sauté over medium-high heat until tender (4-5 minutes). Add cabbage, season to taste and sauté until beginning to wilt (4-5 minutes), add wine and stock, simmer until cabbage is tender and liquid is almost completely reduced (8-10 minutes). Add vinegar, season to taste and keep warm.
Meanwhile, process sage, mustard, garlic, vinegar and 1½ tbsp oil in a small food processor to a smooth paste, season to taste and brush thickly over both sides of each cutlet. Heat remaining oil in a large ovenproof frying pan over medium-high heat, add cutlets and cook, turning once, until golden (2-3 minutes on each side), transfer to oven until just cooked through (4-5 minutes). Serve hot with lentils and red wine cabbage.