Use heavy duty aluminum foil to tightly wrap the bottom and sides of a 9" springform pan, paying special attention to completely cover the bottom and side seams, so that no liquid escapes (you don't want THAT mess). Place the pan on a sheet pan lined with foil or parchment paper and set aside.
Cut 1 medium tomato in half and squeeze gently to remove all the seeds and juice. Dice the tomato, transfer it to a small bowl lined with a paper towel and sprinkle lightly with salt. Set aside.
Heat a skillet over medium-high heat and add 1 pound fresh sausages. Cook until it renders the fat and sears the skin, turning occasionally so each side gets browned, for about 5-8 minutes. Transfer the sausages to a plate to rest. (Note: sausage will not be completely cooked through, but it will finish cooking in) the oven.
Add 1 medium yellow onion to the pan with the sausage fat and sauté for 3-5 minutes, until tender and the onion has absorbed the sausage flavor. Set aside.
Bring a large pot of water to a boil and add a tablespoon of salt to season it; the water will boil even more furiously. Add 1 pound spaghetti and cook, stirring occasionally so the pasta doesn't stick. When it's 2 minutes shy of al dente, remove it from the heat. While the pasta cooks, preheat the oven to 350°F.
Drain the pasta and transfer it back to the same pot it was cooked in. Drizzle with 1½ teaspoons olive oil and toss to coat.
Add 5 ounces fresh spinach leaves to the hot pasta and toss. The pasta will wilt the spinach naturally. When the spinach has wilted, add the diced tomato, sautéed onion, 1 roasted red pepper and ½ pound Swiss cheese. Toss to combine using a pair of kitchen tongs.
Slice the sausages in half lengthwise and cut them into ½" pieces. Add the sausage to the pasta mixture.
In another bowl, whisk 4 large eggs, ¾ cup milk, ¾ cup half-and half, 1 teaspoon kosher salt and ¾ teaspoon black pepper. Pour the mixture over the pasta and toss well with the tongs until well combined.
Spray the springform pan liberally with vegetable spray. Transfer the pasta mixture to the prepared springform pan and smooth out the top. Sprinkle with ⅓ cup Parmesan cheese.
Bake for 30 minutes, then rotate the pan 180° in the oven and continue to bake for 15-20 minutes longer or until pie is solid and no egg juices bubble up from the center when tested with a knife.
Remove the pie from the oven and let rest for 10-15 minutes before removing the foil. Any remaining juices should settle back into the dish. Run a sharp knife around the edges of the pan so the edges of the spaghetti pie don't stick to the collar. Open the collar and carefully remove.
Place a plate on top of the pie and, in one swift move, invert the pan onto the plate. Place the platter you'll serve the spaghetti pie on in the center of the bottom of the pie, then flip it again so the crispy top is right side up. Serve.