Preheat oven to 350°F/180°C and grease a 9×13 inch casserole dish with cooking spray.
Season the chicken with salt and pepper.
Heat olive oil in a skillet, and melt butter. Add the chicken and cook until cooked through, then remove from the pan onto a plate (the chicken won’t get browned and that’s fine).
Add onion and garlic, and cook until the onion is translucent.
Add flour, and cook it for 1 minute then add milk, and gradually add the stock. Use a whisk or a spatula and stir until you get a sauce that’s the same consistency as heavy cream. Season with thyme, and add mixed vegetables and return the chicken and stir.
Transfer the mixture to a 9x13 pan, and arrange the tater tots on top.
Bake for 30-40 minutes or until the tater tots are crispy, then sprinkle the cheese on top and place under the broiler for a few more minutes. Allow to cool on a cooling rack for 15 minutes before serving. Garnish with chopped parsley.