If using crushed tomatoes and you want very smooth ketchup, place all ingredients in a small blender and blend until creamy and smooth. Otherwise, proceed to step 2.
Place all the ingredients in a medium saucepan and stir until well combined, then turn the heat on medium low. Stir gently until everything comes to a light simmer, then cover and reduce heat to low.
Let simmer for 45 minutes to 1 hour, or until slightly reduced and darkened in color, stirring every 10-15 minutes to prevent sticking.
Once reduced, use a teaspoon to remove a spoonful from the saucepan, and let it cool on the counter — you want to taste it cooled for accurate adjustment! Taste and adjust if desired, adding salt to taste, more vinegar (1 Tbsp at a time) for tang, more maple syrup for sweetness, or more onion powder for more savory notes.