Saute sausage in oil in a 10"nonstick ovenproof skillet over medium-high heat until nearly cooked through, about 3 minutes.
Add onion and cook 2 more minutes, or until onion is softened.
Stir in spinach, tomatoes, salt and pepper. Cook 1-2 minutes.
Remove and set aside.
Wipe out skillet and coat generously with nonstick spray.
Combine eggs, cream, and salt in a blender until frothy.
Pour egg mixture into the hot saute pan and cook over medium heat just until curds form, stirring constantly.
Stir in sausage mixture and cheese, transfer pan to oven, and bake 15-20 minutes, or until set in the center.
To unmold frittata, place a large plate, serving side down, over the pan and carefully flip the pan and plate over simultaneously to release. Place another plate on the frittata, invert again, and slice into wedges.