Place oven in center rack. Preheat the oven to 400 degrees F.
Optionally, to cut down on cooking time and make the squash easier to slice in half, you can pre-cook in the microwave. Use a fork or paring knife to poke a few holes around the skin to let steam escape, place in a microwave-safe dish, and microwave for 5 minutes. Carefully remove and let cool for a couple minutes.
Slice the squash in half lengthwise and scoop out the seeds. Rub the cut sides with olive oil and sprinkle with salt and pepper. Place the squash cut-side down on a rimmed baking sheet lined with parchment or foil (spray lightly with cooking spray to prevent sticking).
Bake until the squash flesh is fork-tender and the skin gives a little when pressed. This will take 15-20 minutes if pre-cooked in the microwave, or 40-50 minutes if starting with raw squash. Remove from the oven and set aside to cool. Reduce the oven temperature to 350 degrees F.
Prepare the Filling
Set a large skillet over medium-low heat. Add the sausage and cook, using a spatula to break up the meat into small pieces, until cooked through. (Notes: you can turn the heat up to medium once there’s fat in the pan. Will take around 5-6 minutes, or 10-12 with a nonstick skillet). Drain any excess fat through a fine mesh strainer and discard, then set aside to cool.
In a medium-sized bowl, combine the sausage, poblano pepper, onion, garlic, oregano, parsley, mozzarella, parmesan, crushed tomatoes, salt and pepper.
Stuff the filling into the spaghetti squash, gently packing it in as much as possible. Place back on the same sheet pan (you may need to replace the parchment or foil if a lot of liquid released from the squash). Bake for 25-35 minutes, until the cheese is melted and the filling is hot.
Serve immediately. Leftovers will keep in an airtight container in the refrigerator for 3-5 days.
Make-Ahead: You can prepare the squash and filling, stuff the squash, top with cheese, and then cover and refrigerator for up to 3 days before baking. It might take an additional 5-10 minutes in the oven. Microwave Reheating: If you have leftovers, place them on a microwave-safe dish, cover, and microwave until hot in the center, around 3 minutes.You can get a bit creative with the filling if you like, adding other vegetables you're trying to use up. I think chopped mushrooms would be a great addition!