Preheat your oven to 350 degrees f.
Heat the oil in a large oven-proof skillet over medium heat.
Add the cabbage and sauté, stirring frequently, until it shrinks in size; this can take around 8-10 minutes, and you can add half at first, or as much as the skillet can fit.
Then as it starts to shrink, add handfuls until all the cabbage is used.
Add in the onion and carrots and season with a pinch of sea salt and pepper.
Meanwhile, in a bowl, whisk the eggs together with the pressed garlic, oat flour, and shredded cheddar.
Once the veggies start to get tender and golden in spots, stir in your egg mixture.
Spread out the frittata with a spatula in the skillet, and reduce the heat to a medium-low.
Allow it to cook for around 4 minutes to get a nice golden crust at the bottom.
Transfer your frittata to your preheated oven and bake for 12-15 minutes, or just until the eggs are completely set in the center.
Allow the frittata to cool for a few minutes before serving.
You may slice it directly into the pan or flip it onto a serving platter.
Garnish with chopped green onions.