Preheat the oven to 350°F (175°C).

Mix the ground flax or chia seed with water in a small bowl to create a flax/chia egg. Let it sit for 5 minutes to thicken up.

Sauté the chopped onion and minced garlic in a large skillet with olive oil over medium heat until translucent, about 4-5 minutes.

Add the quartered shiitake mushrooms to the skillet and cook until softened, approximately another 5 minutes.

Combine the toasted walnuts, oats, cooked beans, thawed tofu, miso, nutritional yeast, tomato paste, dried thyme, nutmeg, baking powder, salt, and pepper in a food processor. Pulse until a dough-like mixture forms, approximately 30-45 seconds.

Incorporate the flax/chia egg into the mixture and pulse again to combine everything well.

Form the mixture into meatballs, about 1-2 inches in diameter, and place them on a lined baking sheet.

Bake the meatballs in the preheated oven for about 25-30 minutes or until golden brown, checking at the 25-minute mark.

Prepare the sweet and sour sauce while the meatballs are baking. In a saucepan, combine ketchup, rice vinegar, soy sauce, mirin, sugar, and cornstarch. Heat over medium heat, stirring until thickened, around 5 minutes.

Serve the meatballs with the sweet and sour sauce drizzled on top.