Preheat oven to 425° F. Line a rimmed baking sheet with parchment paper.
Cut cauliflower into florets. Dry well and place on the prepared baking sheet.
Drizzle 1 tablespoon of the olive oil over the cauliflower and toss to evenly coat the cauliflower with the oil. Bake in the preheated oven for 30 minutes, until golden brown on the edges.
Cut the leeks (white and light green parts only) in half lengthwise and then cut into ¼-inch wide half moons. Wash the chopped leeks in a bowl of cool water (or in a salad spinner) and drain well.
In a large Dutch oven or other heavy pot, heat the remaining 1 tablespoon of olive oil and the butter over medium heat. Add the leeks and cook for 4 minutes, stirring occasionally.
Add the garlic and cook for 30 seconds, stirring.
Pour in the vegetable broth. Add the roasted cauliflower, thyme sprigs, salt and pepper. Stir.
Bring to a boil. Reduce the heat to a low simmer, cover and cook for 15 minutes.
Use a fork or spoon to remove the thyme stems (most of the leaves will have fallen off). Carefully transfer the soup to a counter top blender, remove the center piece of the top of the blender and cover with a folded kitchen towel (to allow the steam to vent), and puree until smooth.
Stir in the lemon juice. Taste and season the soup with more salt and pepper, if needed. Serve.