If you don't have a gas grill, you can roast your tomatoes and garlic in the oven. Roast at 425 degrees for about 30 minutes, until tomato skins are blackened and easy to peel. Alternatively, you can also use the broiler setting in the oven. Watch closely and rotate the tomatoes occasionally so they roast evenly.
Instead of a slow cooker, this can also be made in a large pot on the stove. Saute the onions directly in a large 6 quart pot. Add in the rest of your ingredients and puree. Simmer, covered, for 3 to 4 hours over low heat, stirring every 20-30 minutes.
This sauce is great when made a day or two in advance, as this gives the flavor even more time to develop. This sauce can be stored in an airtight storage container (like a mason jar) in the fridge for 7 to 10 days. To freeze, divide the sauce into quart sized freezer bags. Lay the bags flat in the freezer until frozen solid. The sauce will stay good in the freezer for about 3 months.
Sterilize 6 pint sized mason jars. Add 1 tablespoon of lemon juice into each jar. Fill the jars with prepared spaghetti sauce, leaving 1/2 inch at the top. Place the lids and bands on the jars and secure tightly. Process in a boiling water bath for 35 minutes.