Crack all 12 eggs into a large bowl and whisk together with the milk, salt, pepper, nutmeg, dill, and basil until well combined and slightly frothy. The nutmeg adds warmth and depth that rounds out the cheese and greens beautifully. Fold in the cooked shredded chicken gently to distribute evenly. While this sits, prepare all your other ingredients: slice the red onion into thin half-moons, mince the garlic, roughly chop the spinach, and grate or crumble both the Gruyere and feta cheeses into separate piles. Thinly slice the green onions for garnish.

Preheat your oven to 350°F. In a 10-12 inch oven-safe skillet over medium heat, melt the butter and add the sliced red onion. Cook for 3-4 minutes, stirring occasionally, until the onion softens and begins to caramelize slightly—this develops sweetness and depth. Add the minced garlic and cook for about 30 seconds until fragrant, then add the spinach in batches, stirring until it wilts completely and any liquid has mostly evaporated. I like to let the spinach cook down fully before adding the egg mixture because excess moisture will make the frittata watery and prevent proper setting.

Pour the egg mixture from Step 1 directly over the sautéed spinach and onions in the skillet, stirring gently with a spatula to distribute the chicken and greens evenly throughout. Let it cook undisturbed on the stovetop over medium heat for about 5 minutes until the edges begin to set and pull slightly away from the pan sides. Scatter the Gruyere and feta cheeses evenly over the top—I use Boar's Head Gruyere because it has superior melting quality and a nutty flavor that complements the frittata perfectly.

Transfer the skillet to the preheated 350°F oven and bake for 18-20 minutes until the eggs are just set in the center and the cheese is melted and lightly golden. The frittata will continue to cook slightly from residual heat, so remove it when the center still has a slight jiggle—it will firm up as it cools.

Remove the skillet from the oven and let the frittata rest for about 10 minutes—this resting time allows the eggs to set completely and makes it much easier to slice and serve. Top with the sliced green onions just before serving for a fresh onion bite and color contrast. Slice into wedges directly in the skillet and serve warm or at room temperature.