Preheat the oven to 400℉. Line a baking sheet with parchment paper.
Roll out your pie crust on a work surface. Using heart shaped cookie cutters (some medium-sized and some smaller), cut as many heart-shaped pieces as you can and place them on the prepared baking sheet. Brush the hearts lightly with milk, sprinkle lightly with salt, and bake for 10 minutes, or until lightly browned and crisp. Set aside.
PREPARE THE SOUP:
In a medium pot, melt the butter over medium-heat. Add the onion, carrots, celery, potatoes and garlic; cook for 5 minutes, or until the vegetables begin to soften. Season with salt and pepper. Stir in the flour and cook for 1 minutes, or until the flour is no longer powdery and white. Add the thyme and broth. Bring it to a boil, reduce the heat and simmer for 15 minutes, or until the vegetables are tender. Stir in the peas and chicken for the last 3 minutes of cooking. Turn the heat to low, and add the half and half and lemon juice, cooking just until everything is warmed through, adding more broth if the soup is too thick. Season to taste with salt and pepper. To serve, ladle the soup into six bowls. Top with heart-shaped pie crust crackers and thyme sprigs, and serve immediately.