Preheat the oven to 350 degrees.
Brown the sausage in a large cast-iron pan over high heat until it is well cooked, breaking it up with a wooden spoon as you go so that it has the texture of cooked ground meat, about 5 minutes. Remove the sausage to a plate using a slotted spoon, leaving the the fat in the pan; set the sausage aside.
Add olive oil and onion to the pan, reduce the heat to medium, and cook, stirring occasionally, until the onion is very soft and translucent, about 5 minutes; turn off heat.
Whisk the eggs in a large bowl. Whisk in the cream, then mix in the onion, half of the cooked sausage, and about three-quarters of the grated Cheddar. Season the mixture with salt and a little pepper.
Pour the egg mixture into the pan. Sprinkle the remaining sausage over the eggs, top them with the rest of the cheese, and sprinkle tomato in the center. Season everything with salt and pepper.
Cook the frittata over medium heat until you start to see bubbles in the center of the the pan and the edges have barely set, about 5 minutes. Transfer the pan to the oven and bake the frittata 20-20 minutes, until it is fully set and golden brown.
Let the frittata cool for 10 minutes, or to room temperature. Sprinkle it with parsley and cut into wedges. Service with hot sauce if you like.
