Look for a “pastured” label or the Certified Humane seal on your egg carton. Chickens that roam pasture have the most varied diet, so they yield the most nutrient-rich eggs.
1 Preheat broiler with oven rack 6 inches from heat. Heat oil in a 10-inch ovenproof nonstick skillet over medium-high heat. Add mushrooms; cook, stirring occasionally, until browned and tender, 9 to 10 minutes. Add garlic; cook, stirring constantly, for 1 minute. Add kale; cook, stirring occasionally, until kale is tender, about 2 minutes.
2 Beat eggs in a bowl with a fork or whisk until well blended. Stir in Manchego, salt and pepper. Pour egg mixture over mushroom mixture in skillet. Reduce heat to medium and cook until bottom is set, about 3 minutes. Transfer skillet to oven; broil until frittata is set and top lightly browns, about 2 minutes. Serve immediately.