Place a medium pot over medium-high heat and melt the butter. Add in the flour and cook for about a minute. Whisk in the stock and bring up to a bubble. Simmer until thickened, 2-3 minutes, then remove from heat and stir in the mustard, jam, some salt and pepper. Reserve warm while you prepare the brie sticks.
Cut the brie round in half horizontally through the middle, opening it up into two rounds. Cut each round into sticks and reserve.
Set up three dishes for breading: one with the flour, one with the egg and one with the breadcrumbs, parsley and lemon zest. Bread all of the brie sticks by running them first through the flour, then the egg, then the breadcrumbs.