• In a medium casserole or Dutch oven, heat 2 tablespoons of the olive oil until shimmering. Add ½ cup of the onion and 3 of the very finely chopped garlic cloves along with the celery and carrot and cook over moderately high heat, stirring, until the vegetables are softened but not browned, about 5 minutes. Add the canned tomatoes with their juices and all but 1 teaspoon of the chopped basil; season the sauce with salt and pepper and bring to a simmer. Cook the tomato sauce over moderately low heat for 1 hour, stirring occasionally.
• Meanwhile, in a small skillet, heat 1 teaspoon of the olive oil. Add the remaining ¼ cup of onion and 1 chopped garlic clove and cook over moderately high heat until softened, about 4 minutes. Transfer to a medium bowl and let cool. Stir in the bread crumbs, milk and egg white. Add the oregano, 2 teaspoons of kosher salt, ½ teaspoon of pepper and the remaining 1 teaspoon of basil. Add the ground turkey and knead until combined. With lightly moistened hands, roll the mixture into thirty-six 1-inch meatballs and arrange them on a large baking sheet. Refrigerate the meatballs for 30 minutes.
• In a large nonstick skillet, heat the remaining 2 tablespoons plus 2 teaspoon of olive oil until shimmering. Add half of the meatballs and cook over high heat, turning, until browned and cooked through about 6 minutes. Using a slotted spoon, transfer the meatballs to a paper towel-lined plate. Repeat with the remaining meatballs. Add the meatballs to the tomato sauce and simmer over low heat for 15 minutes: season with salt and pepper.
• In a large pot of boiling salted water, cook the pasta until al dente. Drain the pasta well and return it to the pot. Add half of the tomato sauce and toss to coat. Transfer the spaghetti to a large serving bowl and pass the remaining sauce, meatballs and Parmesan cheese at the table.

Make Ahead: The meatballs can be refrigerated in their sauce in an airtight container for up to 2 days.