Preheat the oven to 220 C, 200 Fan, Gas 7. You will need a large paella dish or a shallow circular oven dish about 28 cm in diameter.
Simmer the potato in well salted water for 15 minutes or until tender. Meanwhile, drizzle the garlic with a tablespoon of olive oil and roast in the oven for 15 minutes. Remove from the oven when cooked but leave the oven on.
Sweat the leek in 1 tbsp olive oil over a medium heat for 6-7 minutes until soft but not broken down. Leave to cool.
Break the eggs into a large mixing bowl and whisk together. Season with salt and pepper. Add 1 tbsp basil oil with the spring onions and tarragon. Whisk in the milk. Drain the potato and cool for 5 minutes.
Squeeze the soft pulp from each cooked garlic clove out of its skin into the egg mixture. Beat well then add the cooked potatoes and leeks.
Heat the paella dis on the stove until very hot. Add 2 tbsps olive oil then pour in the egg mixture. Cook for a minute or two on the hob before transferring to the oven.
Cook for 15-20 minutes until the eggs look just set and the frittata is more or less firm to the touch. If the eggs are starting to brown, cover with foil a until the cooking time is up.
If you are feeling brave flip the frittata out on to a plate, or serve frpm the pan. Drizzle with a little extra basil oil and scatter over the parmesan before cutting into wedges.
Serve warm, at room temperature or slightly chilled.