Optional: Roast the strawberries in a 400 F oven for 10 minutes.
Combine all ingredients in a slow cooker. Cover and cook on low for 8 hours.
Purée the ketchup with an immersion blender, food processor, or blender, then return it to the slow cooker.
Keep it uncovered and cook on high for 1 hour to thicken the ketchup. Keep an eye on it, it may be good to stir it once every 20 minutes or so.
Adjust the taste here as needed. If you need tang, try apple cider or balsamic vinegar. For sweetness, use honey or another liquid sweetener.
Transfer to jars and refrigerate or freeze. It'll expand a little while freezing, so leave a little room. It will keep in the fridge for about 2 months, but closer to 6 months in the freezer.