Preheat the oven to 350 degrees. Coat a tart pan or pie dish with olive oil cooking spray.
Cook the whole potatoes in a large saucepan of boiling water for 5-6 minutes, until fork-tender. Drain and let cool. Once the potatoes have cooled, dice them into bite-sized pieces.
Cook the turkey sausage links, then set them aside. Dice the turkey sausage once cooled.
Heat butter or olive oil in a large skillet over medium heat. Add the potatoes and cook for 5 minutes, then add the mushrooms and onion. Cook, stirring often, for another 5 minutes or until the onion and mushrooms are tender.
Add the diced sausage to the potato mixture.
Pour the potato mixture into the tart pan and sprinkle with ½ cup of cheese.
In a large bowl, beat the eggs and milk together, season with salt and pepper.
Pour over the potato mixture in the tart pan.
Bake in the oven for 20 minutes.
Top the frittata with the remaining ½ cup of cheese. Return to the oven for 5-10 additional minutes or until the cheese is melted and the eggs are cooked through.
Remove from the oven and cool for 5 minutes before cutting. Enjoy!