First, heat up a large skillet on medium heat and add the onions when it is hot. Cook for a few minutes or until they are lightly browned on the sides.
Then, add in the carrots and celery and cook for about 2-3 minutes, just to let them soften up a bit.
Add a little more olive oil to the skillet and add in the ground turkey. Fry until the outside has become nicely browned and cooked all the way through.
Next, add the milk and the flour (add flour little by little to make sure it's not too much). Mix everything together well.
Add the potatoes, peas, corn, lentils, seasonings and mix well. Cover and let simmer for about 10-15 minutes or until the potatoes have slightly softened. Preheat the oven to 375°.
Fill a casserole dish (I used a 15”x10”) with the mixture and then gently lay the puff pastry on top, making sure there is enough to completely cover the top and curling over any extra on the sides. Slice a small line in the middle of the dough to help let out any air and then bake in the oven for 30-40 minutes, or until the pastry dough on top has puffed up and become flaky and golden.
Let cool for 10 minutes then slice up and enjoy!