Preheat oven to 350 degrees F. Spray 4 mini cast iron skillets (3.5 inches) with no-stick cooking spray. Place on parchment-lined baking sheet.
Divide the sausage and bacon evenly between the skillets.
Top each skillet with equal amounts of the onions and chopped spinach leaves.
In a mixing bowl, whisk together the eggs, salt, pepper, and half and half. Pour mixture into a mixing cup and gently pour equal amounts into each skillet. Top with cheese sprinkled around the top.
Bake for 25-30 minutes, or until the eggs are set and the frittata pulls away from the edge of the skillet.
Remove from the oven. Top with crumbled bacon. Allow to sit for 5 minutes before running a flat knife along the outside edge of the skillet to remove from the pan.
Gently remove the frittata from the mini skillet and transfer to a plate for serving.