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1
Combine the chuck, half of the chopped onion, the garlic, salt, pepper, bread crumbs, egg yolks, and water in a large bowl.
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2
Mix well and form meatballs that measure about 1-inch in diameter.
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3
Heat the oil in a large nonstick skillet over medium-high heat.
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4
Add the meatballs and cook, shaking and turning, until well browned, about 5 minutes.
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5
Don't crowd the pan; work in batches, if necessary.
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6
Transfer to paper towels to drain.
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7
Pour off any excess fat from the skillet, leaving 3 tablespoons in the pan and add the remaining onion.
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8
Cook, stirring often, until softened, about 5 minutes.
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9
Add the mushrooms and cook, stirring, until the liquid they give off has evaporated, 7 to 10 minutes.
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10
Pour in the sherry, increase the heat to high, and boil until almost all the liquid they give off has evaporated.
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11
Pour in the stock and bring to a boil.
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12
Rub the butter with the flour in a small bowl until it forms a smooth paste.
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13
Pinch off pea-sized pieces and add little by little to the boiling sauce, whisking constantly for 3 minutes.
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14
Add the meatballs, stir in the dill, and sour cream, season with salt and pepper, and cook over low heat until the meatballs are just heated through.
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15
Serve hot.
