Put oven rack in the middle position and preheat oven to 220°C.
Lightly oil a baking dish that will fit the cauliflower. Trim off the outer cauliflower leaves. Cut away the base of the stalk so the cauliflower can sit flat, then cut a cross into its base.
Drizzle 2 tbsp olive oil over the top of cauliflower and sprinkle with 1/2 tsp salt.
Bake cauliflower until tender, about one hour (depending on size). Check by piercing the cauliflower with a paring knife, which should go through easily. Transfer to a serving dish.
Whisk together lemon juice, mustard, capers, parsley and remaining 1/2 tsp salt in a small bowl, then whisk in remaining 1/4 cup olive oil.
Drizzle cauliflower with dressing and present the cauliflower whole, with a butter knife and serving spoon for cutting and serving at the table.