Mom use to make this often and then I started doing it as a holiday tradition with baked Walleye. I recently made it for one of my clients and he exclaimed, 'hey, was that baked Spaghetti?" That was music to my ears.
Put oil in saute pan. Chop the onion, green pepper, and celery. Place in saute pan and sweat for about 15 minutes or till translucent. Add tomatoes. Cook spaghetti. Add cheddar to tomato mixture and simmer till melted through. Lightly coat an 8x8 or 9x9 square pan. Add a layer to bottom of pan. Next add the cheddar mixture. Add another layer of pasta and either add another layer of the cheddar mixture or sprinkle cheddar over the top layer of plain pasta. Bake at 400 till brown.