Heat the oven to 200°C/180°C fan/gas mark 6. Cook the potatoes in a pan of boiling water for 15 minutes, adding the parsnips halfway through until just tender. Drain the vegetables and place on a large baking tray.
To get crispy potatoes, gently press the potatoes down with a potato masher or a fork to slightly flatten them, but so they remain whole. Add the whole garlic cloves and roasting herbs to the potatoes and parsnips. Toss in 1 1/2 tbsp olive oil, season well and bake for 30 minutes until crisp.
Line an 18cm loose-bottomed cake tin with non-stick baking paper. Squeeze the garlic cloves from the skins and mix them into the veg. Tip the vegetables into the tin and press them down so they’re quite tightly packed like a cake. Drizzle over 1/2 tbsp oil and bake in the oven for 30 minutes.
Meanwhile, mix the garlic, remaining oil, ½ tbsp cold water and and lemon juice together, then stir in the parsley to make a dressing. Carefully remove the cake from the tin and serve with parsley oil drizzled over.