Cheddar-Topped Shepard's Pie

I watched Martha Stewart make this on her show and couldn't wait to try this one on my family. It went over very well. My kids gobbled it up, even the veggies inside. It is now part of our regular dinner line-up. Perfect for a potluck.

Ingredients
 2 lbs baking potatoes (about 4) peeled and thinly sliced
salt and pepper
1 Tbsp vegetable oil
6 medium carrots halved lengthwise, quartered if large, and thinly sliced
6 stalks of celery, thinly sliced
1 large onion, chopped
1/2 tsp dried thyme
1/4 cup all purpose flour
1/4 cup tomato paste
2 lbs ground beef chuck
1 cup whole milk
1 1/2 cups shredded sharp cheddar (6 ounces) recipe calls for white cheddar I use whatever I have on hand.

Directions
 Preheat oven to 450 degrees. Place potatoes in a large saucepan, and cover by 1 inch with salted water. Bring to a boil; reduce to a simmer. Cook until potatoes are easily pierced with the tip of a paring knife. (15-20 minutes)

Meanwhile heat oil in a 5 quart dutch oven or heavy pot over med-high. Add carrots, celery, onion, and thyme. Cook, stirring occasionally, until veggies are tender, 8-10 minutes. Add flour and tomato paste; cook stirring 1 minute. Add beef; cook until no longer pink, 6-8 minutes. Add 1 cup water *(I use beef broth if I have it on hand) bring to boil and simmer 1 minute. Set beef aside.

Drain potatoes; return to pan cook over medium stirring until all liquid has evaporated about 1 minute. Remove from heat add milk and 1 cup of cheese. Mash until smooth season with salt and pepper.

Pour beef filling into 13X9inch baking dish. Drop dollops of potatoes on top and spread to edges with a spatula. Use a fork to make decorative peaks and sprinkle with remaining cheese. Bake until topping is browned and filling is bubbling rapidly, about 20 minutes.