Spanish Chickpea Stew with Fennel and Saffron

1 onion, finely diced
4 cloves garlic, finely diced
1 carrot, finely diced
1 fennel bulb, layers pulled apart and cut into cubes
1 teaspoon cumin seeds
1 teaspoon fennel seeds
2 cup vegetable stock
2 x 400-gram tins chickpeas, drained and rinsed
2 x 400-gram tins cherry tomatoes
1 bay leaf
2 pinches saffron threads
¾ teaspoon hot smoked paprika
1 tablespoon fresh thyme leaves
zest of 1 lemon
large bunch silverbeet, trimmed of stalks and cut into thick ribbons (could also use kale or spinach)
sea salt and ground black pepper
3 tablespoons ghee or coconut oil for sautéing 

To serve
fennel fronds to garnish