Step 1

To make the tomato sauce:

Step 2

Purée tomatoes with their juice in a blender and transfer to a bowl.

Step 3

Heat oil in a large heavy pot over medium-high heat and sauté onion until golden, about 6 minutes. Add garlic and sauté, stirring, 1 minute. Add tomato purée, sugar, bay leaf, and 1 1/2 teaspoons salt and simmer, uncovered, stirring occasionally, until thickened, 40 to 50 minutes. Discard bay leaf.

Step 4

Make meatballs while sauce simmers:

Step 5

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.

Step 6

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.

Step 7

Simmer meatballs in sauce and assemble sliders:

Step 8

Add drained meatballs to tomato sauce and simmer, stirring occasionally, until meatballs are heated through.

Step 9

Assemble sliders with a meatball and 1 tablespoon sauce per bun. Garnish with baby greens and secure each with a wooden pick.