Preheat your oven to 350°F (175°C).
In a large bowl, whisk together eggs, milk, salt, and pepper until well combined. Set aside.
Heat olive oil in a 10-inch oven-safe skillet over medium heat.
Add diced onion and cook until translucent, about 3-4 minutes.
Add minced garlic and cook for another 30 seconds until fragrant.
Add spinach to the skillet and cook, stirring occasionally, until wilted, about 2-3 minutes.
Spread the spinach mixture evenly in the skillet and add the halved cherry tomatoes.
Pour the egg mixture over the vegetables, tilting the pan to ensure even distribution.
Sprinkle the cheese evenly over the top of the egg mixture.
Cook on the stovetop for about 5 minutes, or until the edges start to set.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the frittata is set in the center and lightly golden on top.
Remove from the oven and let cool for 5 minutes before slicing.
Garnish with fresh herbs and a pinch of red pepper flakes if desired.
Slice into wedges and serve warm or at room temperature.