Heat a large Dutch oven or pot over medium heat.

Melt the butter and add the sliced leeks. Cook until soft and lightly caramelized, about 5 minutes.

Stir in the minced garlic and cook for 1 minute until fragrant.

Rinse the red lentils in a colander under cold water until the water runs clear, then drain well.

Add the cauliflower florets, diced potatoes, rinsed lentils, thyme, salt, and black pepper to the pot. Pour in the vegetable stock and stir to combine. Bring to a boil, then reduce to a simmer.

Let it simmer for about 25 to 30 minutes, until the potatoes and lentils are tender.

Once cooked, remove from heat. Stir in a splash of milk if desired. Use an immersion blender to partially blend the soup, leaving some chunks for texture.

Ladle the soup into bowls and serve, optionally with a grilled cheese toastie.