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1
Add the beef broth and Worcestershire sauce to a 6-quart slow cooker.
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2
Add the pork, turkey, breadcrumbs, eggs, allspice, onion powder, 2 teaspoons salt and 1/2 teaspoon pepper to a large bowl.
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3
Mix the meat with your hands until the mixture is uniform in color and texture.
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4
Scoop level tablespoonfuls, and roll them into balls; place them in a single layer in the slow cooker.
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5
When the bottom of the cooker is covered, add a second layer, starting on the outer edge.
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6
(The outside of the cooker is hotter; meatballs on the edge will cook more quickly than those in the center.)
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7
Cover and cook on high for 2 hours, until the meatballs are cooked through.
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8
Meanwhile, rub the butter and flour together in a small bowl to make a smooth paste (which will be used to thicken the sauce).
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9
Pinch off 1/2-inch balls, and set on a small plate.
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10
Transfer the meatballs with a slotted spoon from the slow cooker to a large bowl, leaving the cooking liquid in the pot.
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11
(You should have about 1 1/2 cups liquid.)
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12
When removing the meatballs, start from the outer edges and scoop carefully.
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13
Some may be stuck together; gentle coaxing with the spoon will separate them.
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14
Whisk the butter-flour paste into the cooking liquid, 1 or 2 balls at a time, to incorporate.
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15
Whisk in the sour cream and jelly, if using.
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16
Return the meatballs to the slow cooker, and stir with the spoon to coat.
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17
Cook on high, uncovered, until the sauce is thickened, 30 minutes.
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18
Adjust the setting to keep warm, sprinkle the meatballs with the parsley and serve.
