Remove the ugly leaves and roots from leeks. Slice diagonally into 2cm pieces; wash and drain. Slice the knakworstjes into 3cm pieces.
Melt butter in a frying pan. Add onion and cook about 2 minutes. Stir in leeks, ginger, S&P; cook about 1 minute. Stir in knakworstjes; cook about 2 minutes. Stir in apple juice and vinegar, cover, and cook about 5 minutes, till leeks and knakworstjes are cooked. Divide over two plates.
Lekker with stokbrood [or mashed pots, noodles, rice, etc.]