Simmer shallots in boiling water for 5 minutes to soften, then drain.
Fry shallots in butter in a non-reactive pan until golden-brown.
Add sugar and stir until melted and caramelised around the shallots.
Mix wine, port, cassis, and stock; add 1/4 to the pan with bay leaf and lemon zest.
Simmer and reduce liquid to a glaze, then repeat adding remaining liquid until shallots are coated.
Heat 1.5 tablespoons of olive oil and sauté leeks until softened.
Combine leeks, mashed potatoes, and cabbage in a bowl and season well.
Fry spoonfuls of mixture in 7.5cm rings with remaining oil until golden on both sides.