Dice the butter into cubes and add to the bowl of a food processor. Add the flour and salt. Pulse until the mixture resembles fine breadcrumbs. Add the herbs and quinoa and pulse a few times to combine.
Add 4 tablespoons of the water and process for 30 seconds. Add more water, 1 tablespoon at a time, until the mixture forms a ball.
Turn out onto a floured surface, and knead gently. Press into an oblong about 2 cm thick and wrap with cling film. Rest in the fridge for at least 20 minutes.
Melt the butter in a medium fry-pan over a medium heat. Peel and finely slice the onions, then add to the butter. Cook, stirring constantly, for 2 minutes or until the onion is starting to soften. Reduce the heat and cook, stirring every few minutes, for 15-20 minutes until the onion is lightly caramelised. Be sure not to let the onion get crispy or overly-dark
Lightly grease a tart pan. Preheat the oven to 200C bake.
Unwrap the pastry and roll, on a floured surface, to a sheet approx. 1/2 cm thick and big enough to fit the tart pan. Carefully lift and lay into the tart shell, pressing gently to line the bottom and sides. Neatly trim the excess.
Using a fork, poke a few holes in the base of the pastry. Line with a large piece baking paper and add baking weights (or uncooked rice) and blind bake for 12-15 minutes, until the edges are lightly golden.
Remove the tart shell from the oven and carefully lift out the baking paper and baking weights. Return the shell to the oven for 5-7 minutes until the bottom is lightly golden as well. Remove from the oven.
Spread the caramelised onions over the bottom of the blind-baked tart shell. Flake the salmon into chunks and distribute evenly over the onions. Take teaspoons of the cream cheese and dot over the onions as well. Scatter over the capers and finely diced chives.
Crack the eggs into a small bowl and add the milk. Beat with a fork or whisk until well combined. Season with salt and pepper.
Carefully pour the egg mixture over the tart, until about 2ml from the top of the shell.
Bake for 20-25 minutes until the egg is set. Remove from the oven and allow to cool for 10 minutes before removing from the tart pan. Enjoy!
Put 1/4 cup dried quinoa into a pot with 3/4 cup of water and a pinch of salt. Bring to the boil, covered, then turn down the heat and simmer for 15-20 minutes until the quinoa is puffed. Turn off the heat, but leave the pot on the stove. After 5 minutes, remove the lid and drain any surplus liquid from the quinoa. Rinse with cold water, and drain again.
Before using in this pastry, place 1/2 cup of the cooked quinoa into a tea towel, twist into a ball and squeeze out as much moisture as possible.
It may seem fiddly and time consuming to blind bake the pastry, but if you skip this step, the bottom of the tart will be soggy and undercooked. You can, of course, use store-bought pastry in this recipe, but I would still recommend blind-baking.