Toss cauliflower with olive oil and salt and spread in a single layer on baking sheet.
Roast cauliflower until golden brown, about 20 minutes.
Meanwhile, heat butter in a soup pot over medium heat. Add onions and cook until soft and translucent, about 10 minutes. Add roasted cauliflower and chicken stock and bring to a boil. Reduce heat and simmer for 10 minutes.
Remove from heat and let cool.
Transfer to a Vitamix or high-powered blender and puree until smooth. Return to clean pot.
For the grilled cheese croutons: Put two pieces of cheese between two slices of sourdough bread and butter the outside of each. grill in a cast iron skillet or a panini machine, butter side down, until golden brown and cheese melted. Put on cutting board to cool. Remove crusts and cut sandwiches into ½-inch square croutons.
Ladle soup into bowls and top with croutons.
Recipe by Three Hungry Boys at https://www.threehungryboys.com/2018/10/01/1580/