Step 1

Bring 4 cups water to boil in medium saucepan over medium-high heat. Add cashews and cook until softened, about 15 minutes. Drain and rinse well.

Step 2

Heat 1 tablespoon oil in 12-inch nonstick skillet over medium heat until shimmering. Add onions and cook until softened and lightly browned, 8 to 10 minutes. Stir in garlic, oregano, and pepper flakes and cook until fragrant, about 30 seconds. Transfer half of onion mixture to bowl and set aside.

Step 3

Process remaining onion mixture, boiled cashews, 1/4 cup basil, egg, and 1 1/2 teaspoons salt in food processor to fine paste, about 1 minute, scraping down sides of bowl as needed; transfer to large bowl. Add ground beef and knead with your hands until well combined. Pinch off and roll mixture into 1 1/2-inch meatballs (you should have 12 meatballs).

Step 4

Process tomatoes in clean, dry workbowl until smooth, about 30 seconds. Heat 1 tablespoon oil in now-empty skillet over medium heat until just smoking. Brown meatballs on all sides, about 10 minutes; transfer to plate.

Step 5

Add reserved onion mixture and tomato paste to fat left in skillet and cook over medium heat until tomato paste begins to brown, about 1 minute. Stir in processed tomatoes, bring to simmer, and cook until sauce is thickened, about 20 minutes.

Step 6

Return browned meatballs and any accumulated juices to skillet. Reduce heat to medium-low, cover, and simmer gently until meatballs are cooked through, about 10 minutes. Adjust sauce consistency with hot water as needed. Stir in remaining 2 tablespoons basil and season with salt and pepper to taste. (Sauce and meatballs can be refrigerated for up to 3 days or frozen for up to 1 month; gently reheat before serving.)

Step 7

Meanwhile, adjust oven rack to middle position and heat oven to 375 degrees. Using spiralizer, cut zucchini into 1/8-inch-thick noodles, then cut noodles into 12-inch lengths. Toss zucchini with 1 teaspoon salt, 1/2 teaspoon pepper, and remaining 1 tablespoon oil on rimmed baking sheet and roast until tender, 20 to 25 minutes. Transfer zucchini to colander and shake to remove any excess liquid. Transfer zucchini to large serving bowl, add several spoonfuls of sauce (without meatballs), and gently toss to combine. Serve zucchini with remaining sauce and meatballs.