Spiralize the zucchini into noodles; set aside in a large bowl.
Heat 2 TBSP olive oil in a large skillet. Add the diced onions and sauté over medium heat until tender, about 3 minutes. Add the sliced mushrooms and sauté until the mushrooms are tender and golden, about 5-7 minutes. Add the tamari (or soy sauce), the sardines, and the bean puree. With a spatula or large stirring spoon, roughly break the sardines down into small chunks. Bring to a bubbling simmer and let simmer for about 3-5 minutes, stirring often. The sauce should thicken to a rich and creamy consistency. Taste and season with s + p to your liking.
While the sauce simmers, toast the walnuts over medium-low heat in a dry pan until golden brown. This should only take a few minutes. Keep a close eye on them as they can burn up quickly. When done, set aside.
Finally, toss the zucchini noodles into the sauce. Plate, then garnish with toasted walnuts and fresh parsley.