Preheat oven to 350°F (175°C). Position the oven rack in the upper ⅓ of the oven. This will prevent the bottom of the frittata from overcooking while the top sets.

To a 10-inch (25 cm) non-stick, ovenproof frying pan, add 1 teaspoon butter and 2 teaspoons olive oil over medium-low heat. Once the butter is melted and bubbly add the shallots. Cook, stirring until softened, about 5 minutes. Do not brown, adjust the heat as needed. Add the spinach and cook just until it is wilted. Remove the pan and set aside to allow to cool.

Add the eggs to a large bowl and whisk well. Add the 1 cup ricotta, Parmesan, basil salt, pepper and nutmeg. Whisk to help distribute the ricotta. 

Once the shallots and spinach are slightly cooled, mix them into the egg mixture. 

Return the pan to the heat on low. Add the remaining 1 teaspoon butter and 1 teaspoon oil. Pour the egg mix into the pan and use a heatproof, rubber spatula to gently agitate the eggs, as if you're scrambling them. This technique lightens the texture and helps the eggs cook more evenly. As you stir, the eggs will begin to set and release less liquid. You can see this demonstrated in the video below the recipe.

As the frittata starts to set, stir the spinach so it is evenly distributed, then use the spatula to smooth the top.

Transfer to the preheated oven and bake for 5 to 15 minutes (time will vary per oven). After 5 minutes,  rotating the pan halfway so the top gets evenly cooked. And continue to cook until the top is set.

To test for doneness, the center should not look wet, the eggs should look set and should not jiggle too much when you shake the pan and the edges should have pulled away from the edge. The top should not be browned.

Allow the frittata to cool slightly in the pan. This will allow the steam to subside and excess moisture from seeping out. Then top with the remaining ricotta (I use a small ice cream scoop).

To remove from the pan, use a rubber spatula to run around the edge to loosen and lifting slightly so it is not stuck. Slide onto a plate or cutting board.