Preheat your oven to 375°F (190°C). Dice the onion or shallots finely and mince the garlic. Roughly chop the fresh spinach.

Heat the olive oil in a 10-inch oven-safe non-stick skillet over medium heat. Add the diced onion/shallots and cook, stirring occasionally, until softened and translucent, about 3-4 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.

Add the chopped spinach to the skillet. Cook, stirring frequently, until the spinach has completely wilted and most of its liquid has evaporated, about 3-5 minutes. If using, add the halved cherry tomatoes and cook for 1-2 minutes until slightly softened. Remove the skillet from heat and spread the vegetable mixture evenly across the bottom.

In a large mixing bowl, whisk together the eggs, milk, salt, and black pepper until well combined and slightly frothy.

Sprinkle the crumbled feta cheese evenly over the spinach mixture in the skillet. Pour the whisked egg mixture over the vegetables and feta, ensuring it's evenly distributed.

Return the skillet to medium-low heat on the stovetop. Cook for 3-5 minutes, allowing the edges of the frittata to just begin to set. Do not stir.

Transfer the skillet to the preheated oven. Bake for 15-20 minutes, or until the frittata is set in the center and lightly golden on top. An inserted knife or toothpick should come out clean.

Remove the frittata from the oven and let it rest in the skillet for 5 minutes before slicing into wedges. Serve warm.