Spinach Balls with Mustard Sauce

Spinach Balls:

2 10-ounce packages frozen chopped spinach, thawed & squeezed dry Ask a question about this ingredient

2 cups herb stuffing mix, crushed Ask a question about this ingredient

1/2 cup unsalted butter, melted Ask a question about this ingredient

1 cup firmly packed freshly grated Parmigiano-Reggianocheese Ask a question about this ingredient

2 small green onions, finely chopped Ask a question about this ingredient

3 large eggs Ask a question about this ingredient

dashes of freshly ground nutmeg Ask a question about this ingredient

Mustard Sauce (see below) Ask a question about this ingredient

Mustard Sauce:

1/3 cup dry mustard (I use Coleman's) Ask a question about this ingredient

1/2 cup white vinegar Ask a question about this ingredient

1/4 cup sugar Ask a question about this ingredient

1 large egg yolk Ask a question about this ingredient