To make the cashew cream, drain cashews and place in a food processor with all other ingredients. Blitz until smooth and creamy, scraping down the sides of the bowl as necessary. It will take at least 2 minutes for the cashews to break down completely. Add a little more water as needed to get a smooth, creamy consistency. Transfer to a bowl and set aside. If making ahead, store in the fridge; it will keep for 2-3 days.
For the pastry, mix flour and salt together in a mixing bowl. Add olive oil and stir with a fork until it resembles breadcrumbs. Stir in cold water (you may need a tiny bit more; if so, only add 1-2 tsp at a time) until mixture forms a soft dough. Bring dough together with your hands, wrap in plastic wrap and rest in the fridge for 30 minutes. Meanwhile, cook the beetroot and caramelised onions.
Preheat oven to 190°C. Slice beetroot into 5mm-thick rounds. Place on a baking tray lined with baking paper and drizzle with the olive oil, maple syrup and balsamic vinegar, then season with salt and pepper. Roast for 20-25 minutes until tender. Leave oven on.
For the onions, heat olive oil in a large frying pan on medium heat. Add onions and cook until very soft and starting to caramelise (about 10 minutes). Add brown sugar, balsamic vinegar and water, reduce heat and cook until you have a jam-like consistency (about 15 minutes). Stir in thyme and season to taste with salt and pepper. Keep warm.
On a piece of baking paper roll dough out into a large circle about 3-4mm thick. Trim the edges, if you like, for a more uniform look. Prick in a few places with a fork and bake (at 190°C) on a large baking sheet for 16-20 minutes until light golden brown. Remove from oven and top with caramelised onions, spreading them evenly. Arrange roasted beetroot on top. Top with pea tendrils, pine nuts and dollops of cashew cream. Serve rest of cashew cream on the side.