First, sort the mussels. Discard any shell that is cracked, or gaping open and unwilling to close within about 30 seconds of being tapped firmly on the counter. Pull the beard off each one toward the hinge, not toward the opening, and do this right before cooking.

Next, whisk the cornmeal or flour into a very large bowl with 10 cups of cold water. Soak the cleaned mussels for at least 30 minutes and up to 2 hours, which lets them disgorge their sand. Lift them out into a colander rather than pouring, so the grit stays behind, and rinse well.

Then melt the butter in a wide pot over medium heat. Add the shallots, and the garlic if using, and cook until translucent, about 3 minutes. Do not let them color.

After that, pour in the wine and add the parsley sprigs, thyme sprigs, bay leaf and plenty of black pepper. Bring to a boil over high heat and let it reduce for 2 to 3 minutes.

Tip in the mussels all at once and clamp the lid on tightly. Every minute or so, hold the lid down hard and toss the whole pot with a short up and down jerk so the top layer swaps with the bottom layer and everything cooks evenly.

After 4 to 5 minutes the shells will have opened. Take the pot off the heat straight away, because waiting for the last few stragglers turns the rest rubbery.

Finally, stir in the lemon zest. Ladle into bowls with plenty of the broth, leaving the last half inch in the pot in case any sand escaped the soak. Scatter the chopped parsley over the top and serve with French bread.