Preheat the oven to 400°F.
In a bowl, combine the riced cauliflower, egg, and a pinch of salt and pepper. Mix until evenly blended to form your crust base.
Press the cauliflower mixture evenly into a small pie dish or ovenproof skillet to form a thin crust. Bake in the preheated oven for 10 minutes to set.
Meanwhile, heat olive oil in a pan over medium heat. Sauté the diced onion, chopped celery, and carrot until they begin to soften, about 3-4 minutes.
Add the green peas to the pan and continue to sauté for another 2 minutes.
Stir in diced, pre-cooked chicken breast (or lightly sautéed chicken cut into bite-size pieces) and low-fat milk. Season with salt, pepper, and thyme, allowing the mixture to heat through and slightly thicken, about 3 minutes.
Remove the cauliflower crust from the oven, and spoon the chicken and vegetable mixture onto it, spreading evenly.
Return the filled pie to the oven and bake for an additional 8-10 minutes, until the filling is bubbly and the crust has a golden edge.
Let the pot pie cool slightly before serving.