Preheat the oven to 425 degrees F. Roll out the puff pastry into an 11-inch square. Cut out four 5-inch rounds using a 5-inch bowl or ramekin as a guide. Place the pastry rounds on an unlined baking sheet and bake until puffed and golden brown, 15 to 20 minutes; set aside.
Meanwhile, heat 2 tablespoons olive oil in a large pot over medium-high heat. Add the leek and cook, stirring, until softened, about 3 minutes. Add the flour and cook, stirring, 1 minute. Whisk in the milk, 3/4 cup water, 1/2 teaspoon salt and a few grinds of pepper. Bring to a boil, then reduce the heat and simmer until thickened, about 3 minutes. Whisk in the garlic-herb cheese, 2 teaspoons mustard, the lemon zest and 1 teaspoon lemon juice.
Season the cod with salt and stir into the skillet. Simmer gently until cooked through, 6 to 8 minutes. Stir in half the herbs.
Whisk the remaining 1 teaspoon lemon juice, 1/2 teaspoon mustard and 1 tablespoon olive oil in a large bowl. Add the lettuce and remaining herbs; toss. Season with salt and pepper.
Divide the cod mixture among shallow bowls; top each with a round of puff pastry. Serve with the salad.