Baked Eggs with Smoke Mackerel, Celery, & Whole-Grain Mustard

Position a rack in the center of the oven and preheat the oven to 325°F. Butter four, four- to six-ounce ramekins. Set aside a baking pan as deep as the ramekins are tall and large enough to hold the ramekins without them touching each other or the sides of the pan.  Heat the olive oil in a small saucepan over medium heat. Add the celery, toss to coat, cover, and reduce the heat to low. Cook the celery, stirring once or twice, until translucent, about eight minutes. Remove from the heat. Bring the cream to a boil in a saucepan over medium-high heat. Remove from heat. Stir in the mackerel, mustard, and celery. Season lightly with salt and pepper. Divide the mixture among the four ramekins. Add one egg to each ramekin. Place the ramekins in the baking pan, place it in the oven, and carefully pour in very hot water to halfway up the sides of the ramekins. Bake the eggs for 10 minutes. Carefully slide out the rack with the baking pan and sprinkle the cheese on top of the eggs. Continue to bake the eggs for another two to three minutes. Ideally, the whites should be set but tender and the yolks should still be runny. Bake them a few minutes longer if you want a firmer yolk. Sprinkle with chopped parsley. Place two celery leaves on top of each. Serve with toasted bread.