In a large pot set over medium heat, heat the olive oil. Add the shallot and cook until translucent, about 2 minutes. Add the garlic and cook, stirring, for 1 minute. Add the chicken broth, potato, carrots, and celery. Bring to a boil, then lower the heat and simmer, uncovered, for 20 minutes.
Place the flour in a small bowl, then whisk in about 1 cup of the hot broth. Whisk until the mixture is smooth. If the slurry seems too thick, add more broth. Slowly pour the slurry back into the pot, whisking continuously. Bring the soup back to a simmer and cook, whisking regularly, until the soup has thickened, about 5 minutes. Whisk in the Neilson 2% lactose free milk, then stir in the chicken, peas, and lemon juice. Simmer for 2 more minutes. Keep warm until service.
For the puff pastry croutons
Preheat the oven to 175°C (350°F). Line a baking sheet with parchment paper.
Unroll the puff pastry and brush with extra-virgin olive oil. Sprinkle with sea salt and black pepper. Cut the puff pastry into diamonds or squares, or use cookie cutters to create cute shapes. Transfer the cut-out shapes to the prepared baking sheet. Bake for about 20 minutes, or until puffed and golden brown.
Serve the hot soup garnished with chopped Italian parsley and puff pastry croutons.