Unsalted Butter (1/2 cup)
over medium heat in a large pot. Add the
and cook gently, lowering the heat if necessary, until the onions are soft, about 15 minutes.
Tomatoes (10 cups)
Kosher Salt (1 tsp)
to the pot and bring to a simmer over medium-high heat, stirring every so often to ensure the onion isn't scorching on the bottom of the pan.
Once the mixture is simmering, lower the temperature, so the mixture is gently bubbling. Simmer for about an hour, stirring occasionally until the mixture has reduced and feels somewhat thick as you run a spoon through it.
Purée with an immersion blender or transfer mixture to a food processor or blender and purée until smooth. Taste. Add more salt to taste.
Once cool, transfer to storage containers and refrigerate for up to a week or freeze for months.