Place the lentils, hot water and stock in a large, heavy-based saucepan. Bring to the boil over a moderate heat and boil for 5 minutes, skimming off any scum that comes to the surface.
Reduce to a simmer and cook until tender; 20 minutes roughly. Drain and run the lentils under cold water until cool. Transfer to a mixing bowl and combine with the tomato, red pepper, olive oil and balsamic vinegar. Season to taste, then stir in the frisee lettuce and red cabbage.
Adjust the seasoning as necessary, then spoon onto serving plates. Garnish with the red onion, chives and a sprig of rosemary at the top.