Preheat your oven to 350 F.
Whisk the eggs and mustard in a bowl and season to taste.
10 large eggs, Sea salt and freshly ground black pepper to taste, 3 tsp. wholegrain or homemade mustard
Heat the cooking fat in an oven-proof skillet over medium heat. Cook the bacon and onion until the onion is golden (about 5 to 6 minutes).
5 bacon slices, 1 tbsp. Paleo cooking fat or clarified butter, 1 large red onion
Add the spinach to the skillet, and cook for another minute or two or until the spinach wilts.
4 oz. baby spinach leaves
Pour the egg mixture into the skillet. Cook until it hardens just a little and place the tomatoes on top.
2 small ripe tomatoes
Once the frittata is set around the edges but still runny in the center, transfer the skillet to the oven and bake for 30 minutes or until the frittata turns to a nice golden color.
Sprinkle some basil leaves on top and serve.
Fresh basil leaves to taste