Preheat the oven to 450 degrees F.
Begin the recipe by cooking the 4 pieces of bacon first until it is crispy in a large skillet. Carefully remove the bacon from the skillet and cut it into small pieces and set aside.
Next finish chopping the potato or potatoes into small even 1-inch cubed pieces. You will need 1 cup of cut potato. Cut the onion into small pieces as well until you reach ½ cup.
Add the 1 tablespoon olive oil to a heated skillet over medium-high heat. Make sure the skillet is oven safe. Add in the chopped onion and potatoes. Cook stirring frequently, until the potatoes are fork-tender and the onions are translucent, about 5 to 7 minutes.
Add in the 3 cups fresh spinach and cooked bacon to the skillet with the potatoes and onion. Stir until the spinach begins to break down and reduce in volume. Spread the mixture around evenly into the bottom of the pan.
In a small bowl, whisk together the 6 eggs, ¼ cup milk, ¼ teaspoon salt, and ¼ teaspoon ground black pepper.
Slowly pour the egg mixture evenly over the potato mixture in the skillet. Top evenly with the cheddar cheese and continue cooking over medium-high heat, on the stovetop for about 5 minutes, until the eggs begin to firm and set around the edges.
Finish the frittata by placing it in the oven to bake at 450 degrees F. for about 5 minutes. The frittata will be puffy and the edge will be lightly browned and bubbly. Carefully remove the skillet from the oven and serve warm. Enjoy!
Store any leftovers in an airtight container in the refrigerator up to 3 days. Reheat in the microwave until warm before serving.