Cook onions and carrots in oil in large nonstick skillet over medium heat.
Stir and cook for 4-5 minutes until tender. Add mushrooms and bell pepper and stir. Place vegetables into bottom of 4-5 quart crock pot and add remaining ingredients except spaghetti pasta and cheese.
Cover crock pot and cook on low for 7-8 hours.
Uncover, stir thoroughly, then leave cover off crock pot and turn heat to high.
Cook, uncovered, for 1 more hour to thicken sauce.
At this point the sauce can be frozen. Divide into smaller portions and cool in refrigerator.
Wrap, label, and freeze the sauce up to 3 months.
To thaw and reheat, thaw sauce overnight in refrigerator.
Pour into skillet and heat over medium heat, stirring frequently, about 15-20 minutes or until sauce bubbles and is thoroughly heated.
You can even cook the pasta until almost tender and combine it with the cooled sauce. Freeze as directed, and thaw as directed. You may need to add 1/4-1/3 cup water when reheating if freezing the pasta in the sauce.
Or you can go ahead and serve sauce with cooked spaghetti and grated Parmesan cheese. 6 servings.