Melt in a small skillet over medium heat:
Add and cook, stirring until soft, 3 to 4 minutes:
Set aside. Combine in the bowl of an electric mixer and let stand until soft, about 2 minutes:
Add the sautéed onions along with:
Beat on low speed until smooth, then turn the mixer to high speed and beat until light and fluffy, about 2 minutes. Shape the meat into 1- to 1 ½-inch balls. Melt in a large skillet over medium heat:
Brown the meatballs in batches, then remove and drain on paper towels. If the butter in the skillet has burned after cooking all the meatballs, pour it out and discard it, leaving any browned bits in the skillet. Add to the skillet if needed:
Add to the skillet and cook, stirring, until lightly browned:
Slowly add while whisking:
Cook, whisking, until the gravy is thick and smooth. Return the meatballs to the skillet, cover, and cook until the meatballs are heated through, about 5 minutes. Remove them to a platter and whisk into the sauce:
Pour the sauce over the meatballs.