Place 1 cup of tomatoes in a medium bowl with cornstarch; blend with wire whip. Set aside.
Combine remaining tomatoes, onions, peppers, vinegar, sugar, onion salt, chili powder, garlic powder, cayenne pepper and celery seed in a large stock pot or Dutch oven. Simmer on medium low heat 20 minutes. Add corn starch/tomato mixture, stirring constantly to avoid lumps. Continue simmering an additional 10 minutes or until salsa reaches desired constancy.
Fill clean pint jars with hot salsa, leaving 1/2-inch headspace. Wipe rims clean if necessary. Place lids on top, then firmly screw on bands. Process in boiling water canner for 20 minutes.
Ladle hot salsa into pint freezer containers, leaving 1/2-inch headspace. Place lids on containers. Refrigerate up to 3 weeks OR freeze up to 6 months.
Recipe Note: Recipe Note: To easily skin tomatoes, place whole tomato in boiling water for 15 to 30 seconds. Remove with slotted spoon. Skin will easily peel off!