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1
Puree tomatoes with their juice in a blender and transfer to a bowl.
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2
Heat oil in a large heavy pot over medium-high heat and saute onion until golden, about 6 minutes. Add garlic and saute, stirring, 1 minute. Add tomato puree, sugar, bay leaf, and 1 1/2 teaspoons salt and simmer, uncovered, stirring occasionally, until thickened, 40 to 50 minutes. Discard bay leaf.
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3
Make meatballs while sauce simmers:
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4
Stir together bread crumbs and milk in a large bowl and let stand 10 minutes. Add meat, garlic, cheese, eggs, parsley, oregano, marjoram, 1/2 teaspoon salt, and 1/2 teaspoon pepper and blend until just combined well (do not overmix). Form level 1/4-cup portions into meatballs.
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5
Heat oil in a 10-inch heavy skillet over medium-high heat hot and fry meatballs, about 6 at a time, turning occasionally, until well browned and cooked through, 4 to 6 minutes per batch. Transfer to paper towels with a slotted spoon.
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6
Simmer meatballs in sauce and assemble sliders:
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7
Add drained meatballs to tomato sauce and simmer, stirring occasionally, until meatballs are heated through.
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8
Assemble sliders with a meatball and 1 tablespoon sauce per bun. Garnish with baby greens and secure each with a wooden pick.
