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1
Mix beef, oats, egg, garlic, allspice, ginger, Worcestershire sauce, salt, and black pepper together in a medium-sized bowl. Use your hands to combine mixture until consistent in appearance, about 1 minutes. Roll 2 tablespoons of the meat mixture into a ball; repeat to make about 17 meatballs.
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2
Heat a large skillet over medium-high heat. Add tablespoons butter and allow to melt. Add meatballs to the hot skillet and brown, 4 to 6 minutes. Reduce heat to medium, turn meatballs over, and cook 3 minutes more. Transfer meatballs to the slow cooker, reserving grease in the skillet.
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3
Return skillet to medium-high heat. Add 1 tablespoon butter and flour and whisk to combine, about 30 seconds. Gradually whisk in water, beef base, and steak sauce, scraping and breaking up any browned bits from the bottom of the skillet. Bring mixture to a boil, whisking until sauce thickens, about 1 minute. Pour sauce into slow cooker with meatballs.
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4
Cover slow cooker and cook on High for 2 hours or on Low for 4 to 5 hours.
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5
Remove 1/4 cup liquid from the slow cooker and transfer to a bowl. Slowly add yogurt to prevent curdling. Add tempered yogurt to the slow cooker. Season with additional salt and pepper as desired.
