For the grilled cheese croutons, spread mayo on one side of each bread slice. Layer the shredded Cheddar cheese between the full bread slices. In a nonstick sauté pan over medium-high heat, add the sandwich and griddle until nicely browned on each side, about 3 minutes per side.
Remove grilled cheese from pan and cool for 10 minutes under refrigeration.
Slice the grilled cheese into ½-inch cubes. Arrange on a parchment-lined cookie sheet and bake for 10 minutes. Reserve.
For the soup, in a medium saucepan over medium-high heat, combine oil, onion, garlic, rosemary and thyme. Sauté for 10 minutes until onions are translucent.
Add the tomatoes and chicken stock. Bring to a simmer.
Once at a simmer, add the butter, salt and black pepper and then purée in a blender until smooth.
Garnish with grilled cheese croutons and serve immediately.