Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).

Heat one tablespoon oil in a frying pan. Add onion and sauté until translucent, about five minutes, stirring occasionally. Add garlic and sauté an additional two minutes. Set aside.

Pat the zucchini and mushrooms dry before adding to a bowl. Add two to three tablespoons olive oil, thyme, salt, and black pepper, and mix together. Spread in a single layer on a baking pan lined with parchment paper. Place in the oven and bake for 25 minutes, tossing midway through baking time.

Transfer sautéed onion and garlic to a medium-sized pot. Add roasted zucchini and mushrooms, reserving some of the mushrooms for garnish, if desired. Dissolve chicken soup mix in four cups boiling water and pour into pot. Bring to a boil. Add cashews. Simmer for 15–20 minutes, or until zucchini is soft. Remove from heat. Blend until smooth and velvety.

Whisk the eggs, oil, and seltzer in a medium bowl. Gently stir in matzah meal, baking powder, salt and pepper, and parsley if using. Mix just until combined. (Overmixing will cause the kneidlach to be dense.) Cover and refrigerate for half an hour to one hour.

Bring a large pot of water with 1 teaspoon salt to a boil. Using a small cookie scoop, form the kneidlach into little balls and drop into the water. (You can do this by hand as well.) Cook for 20 minutes, covered. Drain and serve with hot soup.