Preheat the oven to 350°F (175°C).
Heat the olive oil in a pan over medium‑low heat, add the finely diced onions and peppers with a pinch of salt and cook, stirring occasionally, until soft, sweet and lightly golden, then set aside to cool slightly.
Add the eggs, cottage cheese, Parmesan, salt, pepper and xanthan gum (if using) to a blender jug (or use an immersion blender in a tall jug or big bowl) and blend until completely smooth, creamy and slightly frothy on top.
Lightly grease a 12‑cup muffin tray with a little olive oil.
Pour the blended egg mixture into a mixing bowl or jug and gently fold in the cooled onions and peppers, chopped spinach, mozzarella, feta and any optional add‑ins like green onion, herbs or red pepper flakes, being careful not to knock out too much air.
Divide the mixture evenly between the muffin cups, filling each about three‑quarters full (makes at least 12 egg bites).
Bake for 20–25 minutes, or until the centres are set and the tops are lightly golden.
Remove from the oven and let the egg bites cool in the tray for a few minutes before gently releasing them; store in an airtight container in the fridge for up to five days and reheat gently or enjoy cold as a quick, high‑protein breakfast or snack.