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1
Make sauce: In a 5- to 6-qt. pan, cook oil and garlic over medium-low heat until garlic softens, 5 to 6 minutes. Add tomatoes, salt, and pepper. Cover, bring to a boil over high heat, then reduce heat and simmer, stirring often, until thick, 45 minutes to 1 hour. If needed, crush tomatoes with a spoon to break up. Stir in basil sprigs. Turn off heat; keep warm.
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2
Meanwhile, make meatballs: Heat 1 tbsp. oil in a large frying pan over medium heat. Add onion and cook until tender, 5 to 8 minutes. Stir in garlic and cook until fragrant, about 1 minute more. Add mushrooms, fennel, salt, and pepper. Cook until mushrooms are tender, 5 minutes. Remove from heat and stir in 1/4 cup wine and the bread until liquid is absorbed.
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3
Transfer mixture to a food processor and pulse to finely chop. Scrape into bowl of a stand mixer and let cool. Add parsley, meats, and 1/3 cup wine and beat on low speed until well blended, 1 to 2 minutes.
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4
Using wet hands, shape meat into 1 1/2-in. balls. Heat 2 large frying pans over medium heat with 1 tbsp. oil each. Brown about a third of meatballs in each pan, turning once and adding oil if needed, 6 to 8 minutes per batch. With a slotted spatula, transfer meatballs to a platter. Repeat with remaining meatballs and oil.
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5
Scrape all the drippings into 1 pan. Whisk in flour, then cook over medium heat until bubbling, 1 to 2 minutes. Whisk in 2 cups sauce to loosen browned bits. Scrape into pan with rest of sauce and stir.
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6
Return sauce to a simmer. Gently stir in meatballs; simmer, covered, until flavors are blended, about 20 minutes. Discard basil sprigs. Cut remaining basil leaves into fine slivers and stir into sauce. Meanwhile, cook spaghetti as package directs.
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7
Drain pasta and transfer to a large shallow bowl. Ladle all the meatballs and about half the sauce on top, and toss to coat. Serve with cheese and extra sauce.
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8
*Crack fennel seeds with a mortar and pestle, or buzz in a clean coffee grinder.
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9
Make ahead: Chill sauce and meatballs up to 1 day, or freeze up to 1 month.
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10
Note: Nutritional analysis is per serving.
