Homemade pasta sauce is a great slow cooker recipe. Use it for spaghetti, lasagna, chicken parm, baked ziti, stuffed shells, maybe even as a pizza sauce if you don't mind it a little chunky.

This isn't an exact recipe, because my girlfriend always plays it by ear but it usually goes something like this:
Brown up a couple pounds of ground beef (or other meat) and drain the excess fat.
Dump the meat into the crockpot/slowcooker, add in a couple cans of tomato paste, mix it up, add a big can of tomato juice (shake it first), and mix again.
That's the basic part done, and from there you season it and add extra tomato products as you like to bulk it up.
Italian seasoning blend, garlic powder, and minced onion/onion powder. 'Too much' is not in my girlfriend's vocabulary for this. Usually a generous dusting covering the whole pot full of each one.
Then she would usually finish it by adding in cans of plain tomato sauce to fill it up, but I've been getting her to add cans of flavored diced tomatos or stewed tomatos.
Fire roasted diced, olive oil+Oregano+garlic diced, italian stewed. They're all good in this, and the slowcook time will cause them to basically melt away in the pot anyway.
Last thing is a sprinkling of sugar. It helps cut the acidity of the tomato. Something like half a tablespoon.
We make that up in the late evening (like 9-11pm), then we leave it to slowcook on low overnight. So simmering for like 12-16 hours.

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@WretroughWorld: Great let us know which ones you made and how you liked it!