Spread the dip in a wide gratin dish–use one that’s going to give you a shallow but wide surface for the dip, so it’s not layered too deeply.Save the remaining clam juice for use in pasta or chowder. I like it when it’s longer a thick paste, but is more scoopable and spreadable. The panko will absorb most of the liquid in the pan, so stir in 1/4 to 1/2 cup of the reserved clam juice until the dip loosens to your preferred consistency.Cook until the liquid in the skillet comes to a simmer, then whisk in the flour, lemon zest, 1/2 cup panko, and 1 teaspoon Parmesan.
Cook for 1-2 minutes more to soften the garlic. Keep an eye on them to make sure they don’t start to brown too much. Cook for 5-7 minutes, stirring frequently, until the onions are very soft.
