Cut the zucchinis in half down the length, then scoop out the middle seedy part of the zucchini using a melon baller, discarding the scooped middle part.
Place the prepped zucchinis on a sheet pan, and sprinkle with salt and pepper.
Heat a skillet over medium high heat for a few minutes, then add the sausage to the pan. If the pan is properly hot, it should hiss and sizzle when you add the sausage.
Cook the sausage for about 5 minutes, breaking up any large clumps with your cooking utensil, until the sausage has browned and cooked through.
Remove the sausage to a large bowl, then add the bread crumbs, parmesan cheese, garlic, chopped tomatoes, egg, parsley, and red onion.
Add 1/4 tsp each of salt and pepper, then mix it all together.
Scoop the filling into the prepped zucchinis, then roast in the oven for 15-20 minutes, until the zucchini is tender but not mushy. Enjoy!