Heat your cast-iron skillet with avocado oil (or any other oil / butter with a high smoke point); make sure you add enough oil or butter to cover the entire skillet. You'll want your stove on high.
Turn down the heat to medium and toss in your frozen sausages and onions. Sauté until the onions are translucent. Add in your tomatoes and banana peppers. Mix thoroughly.
Crack your eggs into a bowl and whisk. Add your spinach, nutritional yeast, basil, sea salt, black pepper, and chili flakes to the eggs. Combine.
Before pouring your egg mixture into the skillet, take a spatula and dice up the sausages into bite-sized pieces. Then, add in your eggs.
Cook the eggs on medium for five minutes, then cover the skillet with a piece of tin foil. Let the eggs cook for another 5-10 minutes, or until the middle is firm.
Remove from the heat, slice into equal fourths (or eighths), and serve.
Store the remaining frittata slices in the fridge, covered, for a few days.