Preheat the oven to 180C/350F/Gas 4.
For the baked sea bass, season the fish well with salt and freshly ground black pepper and place on a baking sheet. Drizzle over the olive oil and lemon juice and bake in the oven for 6-8 minutes, or until cooked through.
For the creamed leeks, heat the butter in a frying pan and gently fry the leeks for five minutes, until they start to soften.
Add the cream and simmer for about 4-5 minutes, until the leeks are soft and the cream has reduced slightly.
For the tomato and mushroom sauce, heat the oil in a pan and gently fry the garlic and mushrooms for 4-5 minutes. Add the tomatoes and simmer for a further 3-4 minutes. At the end of cooking, stir in the lemon juice.
To serve, spoon a pile of the creamed leeks into the centre of a serving plate. Place the roasted bass on top and drizzle over the tomato and mushroom sauce.