Preheat your oven to 400°F (200°C). Toss the butternut squash cubes with a tablespoon of olive oil, salt, and pepper, then spread them on a baking sheet. Roast for about 20-25 minutes until tender and lightly caramelized, filling your kitchen with a sweet, nutty aroma.
2 cups butternut squash cubes
While the squash roasts, heat a tablespoon of olive oil in your skillet over medium heat. Add sliced onions and cook slowly, stirring occasionally, until they turn deep golden brown and smell sweet — about 15 minutes.
1 large yellow onion
In a mixing bowl, whisk together the eggs, milk, chopped sage, salt, and pepper until well combined. This mixture will become the custard base for your frittata.
6 large eggs, 1/4 cup milk, 1 tablespoon fresh sage leaves, 2 cups butternut squash cubes
Once the onions are caramelized and the butternut squash is roasted, gently fold the squash and caramelized onions into the egg mixture, ensuring they are evenly distributed.
2 cups butternut squash cubes, 1 large yellow onion
Pour the entire mixture into the skillet that has the remaining tablespoon of olive oil, spreading it out evenly. Cook over medium heat for about 3-4 minutes until the edges start to set and turn golden.
Transfer the skillet to your preheated oven and bake for 15-20 minutes until the frittata is puffed, firm, and golden brown on top. You’ll see tiny bubbles in the center when it’s ready.
Remove the skillet from the oven and let the frittata rest for a couple of minutes. Use a spatula to loosen the edges before slicing.
Slice the frittata into wedges and serve warm, garnished with extra sage leaves if desired. The surface should be crispy with a fluffy, custardy interior filled with tender squash and caramelized onions.
