Preheat oven to 375 degrees F (190 degrees C). Line a baking sheet with aluminum foil.
Season squash halves with olive oil, salt, and black pepper. Place squash, cut-side down, on prepared baking sheet.
Bake in the preheated oven until tender, about 1 hour. Cool squash for 10 minutes; scrape flesh out of squash with a fork.
Cook and stir sausages in a skillet over medium heat until no longer pink in the center, 7 to 10 minutes. An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C).
Stir spaghetti squash, canned tomatoes, diced tomato, and garlic into skillet with sausages; cook and stir until warmed-through, 2 to 4 minutes. Add spinach; cook and stir until spinach wilts, 2 to 4 minutes. Season with salt and black pepper.