LEEK, CANNELLINI BEAN AND HAM STEW

500g leeks, sliced • 10g unsalted butter • 2 tbsp olive oil • 2 garlic cloves, finely sliced • 1 fennel bulb, trimmed, fronds reserved • ½ tsp fennel seeds • ¼ tsp chilli flakes • 2 x 400g cans cannellini beans • 500ml chicken stock • 1 bay leaf • 180g pulled ham hock • 1 tbsp cider vinegar, plus extra if needed