In a medium bowl, whisk the eggs and milk until frothy.
In a large ovenproof skillet, heat 2 teaspoons of the olive oil over medium-high heat. Add the chicken sausage coins and brown them on all sides, 4 to 6 minutes, turning often. Transfer the sausage to a plate until ready to use.
Add 1 tablespoon of the olive oil to the skillet and heat it over medium-high heat until shimmering. Add the onion and 1/2 teaspoon of the salt. Cook, stirring often, until the onion is soft but not browned. Add the garlic and cook, stirring often, until fragrant, about 1 minute. Add 1 tablespoon room temperature water to the pan to stop the garlic browning, and the remaining tablespoon of olive oil, then stir in the spinach and the remaining 1/2 teaspoon of the salt. Cook, stirring often, until the spinach is wilted and the water in the pan evaporates, about 2 minutes. Spread this frittata filling evenly in the skillet and layer with the chicken sausage.
Pour the egg-milk mixture over the frittata filling and use a silicone spatula to scrape the sides of the pan and gently pull eggs from the edges toward the middle so that soft curds form and the frittata begins to firm up.
Give the pan a couple of good shakes to release the frittata from the pan bottom (the top of the frittata will still be raw). Sprinkle the olives and feta over the eggs. Transfer the skillet to the oven and cook until the fritatta is mostly dry (the feta will release some liquid--this is fine--but just be sure the egg is completely cooked) and lightly browned on top, 5 to 7 minutes.
Let rest 5 minutes, then slide the frittata onto a large plate and sprinkle with flaky sea salt and pepper to taste. Top with more feta, parsley and olive oil. Slice into wedges and serve warm or at room temperature.