Preheat oven to 350 F (191 C).
Heat a little olive oil in a heavy bottomed oven-safe pan over medium heat and add the sausage meat, casing removed, breaking it up as it cooks. Cook until the pink is gone, then toss in the mushrooms. Continue to cook until sausage and mushrooms are cooked through.
In a medium bowl, whisk together the eggs, cream, Gruyere cheese, and a pinch of salt and pepper.
Once the sausage and mushrooms are cooked, turn off the heat and pour in the egg mixture, carefully stirring to distribute all the ingredients.
Gently scatter the red onion over the top, followed by the zucchini. Transfer the pan to the oven, and bake for 25-30 minutes, or until the frittata is puffy and the center is cooked. You can check with a knife or toothpick.
Remove from oven and let rest for 10 minutes. Top with chopped parsley and/or dill and serve with a side salad.
The frittata can be kept in the refrigerator for up to five days.