Preheat the oven to 350 °F (177°C).
Whisk the eggs, cream, and green onion in a large bowl. Season with thyme, salt, and pepper.
8 whole eggs, ¼ cup heavy whipping cream, 2 tablespoons green onion, 1 teaspoon dried thyme, 1 teaspoon salt, ½ teaspoon ground black pepper
Add butter to an oven-proof skillet over medium heat on the stovetop. Once it melts, sauté minced garlic for 1 minute, then add the spinach and stir until it begins to wilt.
1 tablespoon butter, 2 cloves garlic, 2 cups spinach
Remove the spinach from heat and combine it with the eggs in a bowl. Crumble the feta cheese with your hands into the egg mixture and stir until combined.
1 ½ cups feta cheese
Add more butter to your pan, if needed, so nothing sticks. Pour the egg and spinach into the skillet. Use a fork or spatula and move the spinach around until it's distributed evenly throughout the frittata.
Bake for 15-20 minutes until the top turns golden brown and the edges set.