In a skillet (cast iron, preferably), cook the sausage over medium heat until crumbly and cooked through. Scoop the meat out and set aside, reserving the leftover grease in the skillet.
In a large bowl, whisk together the eggs and heavy cream until combined. Stir in 1/2 cup of the cheddar, and add a bit of salt and pepper.
Chop the peppers to your desired size. Heat the same skillet you used for the sausage with the leftover grease over medium heat, and add the peppers, spinach, and tomatoes. Cook until the peppers have softened and the spinach has wilted, about 7-8 minutes. Stir in the sausage and toss to combine.
Make sure the ingredients in the skillet are evenly distributed. Slowly pour in the egg, cream and cheese mixture, trying to evenly cover the peppers and sausage. Once poured, quickly use a spatula to distribute the egg to the edges of the skillet if need be. Let the frittata cook in the skillet over medium heat for about 3-4 minutes, or until the edges start to set a bit.
Sprinkle the top with the remaining cheddar cheese and place in an oven heated to 375 degrees Fahrenheit for about 20 minutes, or until the frittata is set in the center.