Preheat the oven to 350°F (180°C).
Line a baking sheet with a silicone liner or parchment paper.
Make the flax egg: In a small bowl, whisk the filtered water and flax meal together until well combined. Set aside to allow the gel to form.
Place the drained walnuts into a food processor fitted with the s blade, and pulse until finely ground. (You do not want chunks of walnuts or your veggie balls will be crunchy.) Add the blanched almond flour, mushrooms, red onion, parsley, nutritional yeast, Italian seasoning, tomato paste, balsamic vinegar, mustard, garlic powder, salt, lemon juice, red pepper flakes, and flax egg, and process until well combined and the consistency of pâté.
Using a tablespoon measurer, scoop out the mixture, and with your hands, roll into 20 equal-sized balls. (The mixture will feel wet, but it will hold together when cooked.)
Transfer the balls to the prepared baking sheet. Using a pastry brush, gently glaze each veggie ball with a tiny amount of olive oil. (Alternatively, spray them with cooking spray.)
Bake for 15 minutes. Then, brush or spray again with oil. Bake for a further 15 minutes, until cooked through, slightly browned, and firm on the outside. Allow the balls to cool on the tray for 5 to 10 minutes before using.
Serve out one portion (5 veggie balls), and transfer the remaining balls to a sealed container in the fridge or freezer for future meals or snacks.