Crock-Pot (or Stovetop) Tomato Soup

Bahan-bahan (12 porsi)

• 84 ounces crushed tomatoes ((3) 28-ounce cans)
• 1 large yellow onion (chopped small, about 1½ cups worth)
• 2 cups water (OPTIONAL)
• 6 tablespoons butter
• kosher salt, to taste (only as needed)