Preheat your oven to 400°F (200°C). Toss the diced sweet potatoes with 1 tablespoon of olive oil and spread them out on a baking sheet. Roast for about 20 minutes until tender and slightly caramelized, filling your kitchen with warm, sweet aromas.
While the sweet potatoes roast, heat the remaining olive oil in an oven-safe skillet over medium heat. Add the chopped onion and sauté until they become soft and translucent, about 3-4 minutes, with a gentle sizzle and fragrant aroma filling the air.
Add the minced garlic and chopped sage to the skillet with the onions. Cook for about 30 seconds until fragrant, with the mixture sizzling lightly and the herbs releasing their aroma.
In a mixing bowl, whisk together the eggs, milk, cinnamon, salt, and pepper until the mixture is smooth and slightly frothy, ready to embrace the sautéed vegetables and roasted sweet potatoes.
Add the roasted sweet potatoes to the skillet with the onions and herbs. Gently fold them together with a spatula, allowing the sweet potatoes to warm through and mingle with the aromatic mixture.
Pour the beaten egg mixture evenly over the vegetables in the skillet. Let it sit for a minute, then gently shake the skillet to distribute the eggs and ingredients evenly, ensuring good coverage and a cohesive base.
Transfer the skillet to the preheated oven and bake for about 12-15 minutes until the eggs are puffed, set, and lightly golden on top. You may see bubbles form as the eggs cook through, signaling readiness.
Remove the skillet from the oven and let the frittata rest for a few minutes. The edges should be crispy, and the interior tender and fluffy, with the surface slightly browned and aromatic.
Slice the frittata into wedges and serve warm, perhaps with a simple green salad or crusty bread to enhance its rustic charm and warm flavor profile.
