Preheat the grill to its highest setting.
For the roasted Portobello mushrooms, mix the garlic with the butter in a small bowl, then spread over the top of the mushrooms. Season with salt and freshly ground black pepper and place under the grill for six minutes.
To make the mushroom sauce, heat the olive oil in a small saucepan and add the leek. Sauté for 2-3 minutes, until lightly browned. Add the garlic and mushrooms and sauté for a further two minutes.
Stir in the double cream, add the Dijon and season with salt and freshly ground black pepper. Cook gently for one minute. Finally stir in the parsley and keep warm until ready to serve.
To make the broccoli salad, steam the broccoli florets for six minutes, or until al dente. Toss in the olive oil and season with salt and freshly ground black pepper.
To serve, arrange the broccoli florets around the plate, pour the mushroom sauce into the middle of the plate and top with the grilled Portobello mushrooms.