Heat the olive oil in a large saucepan, and add the onions, leeks, chopped tomatoes, and garlic. Cook and stir over a low heat for 8-10 minutes until all vegetables are soft.
Stir in the fennel, bay leaf, and orange zest. Season to taste with salt and black pepper. Turn up the heat to high, add vegetable stock (Store-bought or home), and bring to a simmer for about 3-5 minutes to allow the oil and water to emulsify.
Taste the bouillabaisse and adjust the seasoning. Stir in saffron.
Remove from heat and serve immediately, garnishing each place with some reserved fennel fronds.