Start by melting the butter in a large soup pot. Once melted, add in the onion and cook until they become nice and tender. Once tender, add in the garlic, and toss in the flour, and stir, cooking for a few minutes to cook out the flour. Next, add in the milk, and whisk until you have a nice and smooth sauce.
Add in the chicken stock, and give another stir. Reduce the heat to a medium-low, and continue to cook for about 20 minutes or so. Add in the thyme.
Next, add in the broccoli, carrots, and kale. Cook for about another 20 minutes or so until the vegetables get nice and tender. Now we are going to blend. Pour the mixture into a large blender, cover, and then cover with a towel. You can do this in batches as well if you have a smaller blender. Otherwise, if you have an immersion blender, use that.
Pulse until you have a nice puree, then return the mixture back to the pot. Continue cooking on low heat, and then add in the cheddar cheese. If the soup if too thick for you, then feel free to add in about a cup of water or so to thin it out a bit. Give a good stir, and serve with croutons on the top.