6 ounces/170 g yellow-fleshed sweet potatoes, cut into 1⁄2-inch/1.25 cm dice
2 medium-size leeks, white and light green parts only, quartered lengthwise and cut into 1⁄2-inch/1.25 cm slices
12 large organic, pasture-raised, or antibiotic-free eggs
1⁄2 cup/120 mL grass-fed organic milk or unsweetened nut milk
4 teaspoons chopped fresh thyme
11⁄2 teaspoons fine sea salt
1 teaspoon freshly ground black pepper
1 cup/125 g grated Parmesan cheese, divided
2 cups/30 g loosely packed spinach leaves
In a medium, ovenproof skillet over medium heat, heat the oil. Add the potatoes and cook, stir- ring occasionally, for 2 minutes. Add the leeks and cook, stirring occasionally, until all the vegetables are tender, about 6 minutes.
Meanwhile, in a medium bowl, whisk the eggs. Whisk in the milk, thyme, salt, pepper, and 3⁄4 cup/90 g of the cheese.
Add the egg mixture to the skillet, stirring gently to evenly distribute the vegetables. Lower the heat to medium-low and cook without stirring until the eggs are set around the edges, about 10 minutes.
Meanwhile, preheat a broiler and arrange a rack about 8 inches from the heating element.
Scatter the spinach leaves on top of the frittata, pressing them down to slightly submerge, and sprinkle on remaining 1⁄4 cup/35 g of cheese. Place the skillet under the broiler and broil until the eggs are set and the top of the frittata is golden brown, about 5 minutes.
Transfer the frittata skillet to a wire rack (careful, the handle and pan will be hot) and let cool for 10 minutes.
Cut the frittata into eight to ten wedges. Serve warm or room temperature.