Preheat the oven to 350 degrees F.
Put the squash in a 12-inch high-sided skillet, cover halfway with water and bring to a simmer. Turn to low, cover with a lid and cook until tender crisp, about 8 minutes. Set the squash aside to cool.
Drain the water from the skillet, wipe dry and place over medium-high heat. Add the oil and once hot, stir in the turkey sausage. Cook, stirring, until almost cooked through, about 4 minutes. Stir in the onions and cook until softened, about 4 minutes more. Toss in the tomatoes, season with a big pinch of salt and pepper and cook until the tomatoes start to break down a bit, 1 more minute. Set aside to cool.
While the mixture is cooling, trim the ends of the squash, slice in half lengthwise and scoop out the seeds with a spoon, making a boat shape. Chop up the centers and combine with the sausage filling.
Spoon the mixture into the shells, mounding them high, and place on a baking sheet. Mix together the panko, Parmesan and chopped parsley in a small bowl and sprinkle on top of the stuffed squash. Drizzle everything with olive oil and bake until golden on top, about 25 minutes.