In an oven-proof skillet or frying pan, heat the olive oil over medium heat. Once hot, add the diced onion and sauté for about a minute until it becomes translucent.
Add the spinach to the pan and sauté until it reduces in volume.
Preheat your oven to 400 degrees F.
Wash the asparagus and trim off the hard ends. Cut the remaining stalks into 1 to 1 and a half-inch long pieces.
Peel and mince the garlic. Once the spinach has wilted, add the garlic and the chopped asparagus to the pan and sauté for another 1 to 2 minutes.
In a medium bowl, beat the eggs. Add salt, black pepper, thyme, and milk, and stir until combined.
Pour the egg mixture over the sautéed vegetables in the skillet. Allow it to cook for a few minutes on the stovetop, or until the edges start to set.
Transfer the skillet to the preheated oven and bake for about 15 minutes or until the center is fully set.
Once baked, remove the frittata from the oven and let it cool slightly before slicing. Serve warm or cold.
