1. 1. In a large bowl, whisk together the eggs, milk, sourdough starter discard, salt, and pepper until well combined and slightly frothy. Set aside.

2. 2. Preheat your oven to 375 F (190 C). Place a 10-inch cast iron skillet over medium heat on your stovetop and add the olive oil. Allow the skillet to heat up for a minute or two; you should see the oil shimmer slightly.

3. 3. Add the chopped onion to the hot skillet and cook for 3-5 minutes, stirring occasionally, until it becomes translucent and softened. Then, add the minced garlic and cook for another minute until fragrant, being careful not to burn it.

4. 4. Add your chosen chopped winter vegetables (butternut squash, kale, Brussels sprouts, etc.) to the skillet. Cook for 8-10 minutes, stirring occasionally, until the vegetables are tender-crisp. If using denser vegetables like butternut squash, they should be fork-tender.

5. 5. Spread the cooked vegetables evenly across the bottom of the skillet. Pour the egg mixture over the vegetables. Sprinkle half of the shredded cheese evenly over the top of the egg mixture.

6. 6. Let the frittata cook on the stovetop for 2-3 minutes until the edges just begin to set. Then, carefully transfer the skillet to the preheated oven. Bake for 15-20 minutes, or until the frittata is puffed, set in the center, and lightly golden brown. A knife inserted into the center should come out clean.

7. 7. Once baked, remove the skillet from the oven (use oven mitts!). Sprinkle the remaining cheese over the top and let the frittata rest for 5 minutes before slicing and serving. This resting period allows the frittata to set completely and makes it easier to cut.