Line a baking sheet with parchment paper. Arrange potato slices in a thin, slightly overlapping layer. Top evenly with bacon and shallot rings. Drizzle with olive oil and season with salt and pepper. Bake 20 minutes or until vegetables are cooked through and bacon is lightly crisp. Remove from oven and set aside.
Break the eggs into a medium bowl. Add milk and whisk until mixture is very well combined. Season with salt and freshly ground black pepper (take care to not over-season, keeping in mind that the potato mixture is seasoned and the cheese will add salt as well). Set aside.
Reduce oven temperature to 350˚F.
Heat a 10", well seasoned, cast iron skillet over medium high heat until hot.
Add avocado oil to the pan and swirl to coat. Layer potatoes, bacon, and shallots over the bottom; pour egg mixture over all. Remove from heat and sprinkle with cheese.
Bake 25 minutes, or until the frittata has puffed and the center is set.
Let cool 10 minutes before slicing in to wedges. Serve.