Melt butter in a pan over medium heat. Add onion and soften until translucent but not browning, around 5mins.
Meanwhile chop broccoli and pumpkin - use whole stem not just florets of broccoli - in small to medium dice. Add to pan and sweat for around 10 minutes, stirring occasionally and covering in between.
Add thyme, season with salt and pepper to taste and add stock.
Bring to boil and simmer for 5-10mins until the vegetables are tender.
Take off heat and blend, easiest with hand blender or transfer in batches to blender.
Add grated cheddar and stir in, allowing the heat to melt the cheese into the soup.
Add milk or cream and serve.
Notes
I find the easiest way to blend soups like this is with an immersion/stick blender (affiliate link).