Heat up the oil and add the onion and garlic once hot.
Fry for around 10 minutes until softened.
Add the mushrooms and fry for a further 5 minutes until softened.
Place the cooked onion, garlic and mushrooms in a food processor along with all the other ingredients, and blend briefly until just combined - you'll want it to be easy to stick together but still retaining some texture.
Taste and adjust seasoning if necessary.
For the pastry:
Preheat the oven to 180 degrees Celsius (350 degrees Fahrenheit).
Mix together the coconut oil, ground almonds, gluten-free flour and water in a bowl until it forms a firm dough - add the water a few tablespoons at a time so that you don’t add too much!
Lay out a sheet of baking paper on a counter. Sprinkle the surface of the paper and a rolling pin with a generous amount of flour.
Take the pastry dough and use the rolling pin to roll it into a square shape, as thin as you can without it breaking.
Use your hands to shape the filling into a long sausage shape that’s just a little shorter than the pastry.
Place this on the pastry, about a third of the way in from one side, making sure to leave a little bit of space at the top and bottom of the pastry sheet.
Pick up the baking paper on one side and use it to roll the pastry crust over the top of the sausage shape.
Do the same for the other side, then tuck in the top and bottom of the pastry.
Line a baking tray with baking paper and carefully pick up the wellington with both hands and place it upside down on the tray, so that the nice smooth part of the pastry is on top.
Mix together the agave syrup and almond milk in a small bowl, and use your fingertips, a pastry brush or a small spoon to spread it over the tops and side of the wellington.
Sprinkle over mixed seeds, if desired.
Bake in the oven for 35-40 minutes, until the pastry crust is firm to the touch and golden brown.
Tastes best when fresh, but keeps covered in the fridge for up to a couple of days.