Heat oven to 400°F. Peel the onion, cut it in half and then into thin slices. In a 4-quart sauce pot, warm 2 tablespoons oil over medium high heat until hot but not smoking. Add the onion, season with salt and cook until tender and caramelized, about 10 minutes. While the onion cooks, prepare the vegetables.

• Cut the potato into small cubes.
• Peel the carrot and cut into small cubes.
• Strip the leaves from the thyme sprigs.
• With the side of a knife, smash the black garlic to a coarse paste.

Add the potato, carrot, thyme, and black garlic to the onions. Season with salt and cook until the vegetables begin to soften, about 5 minutes. Add the lentils, 4 cups water, bring to a boil, reduce to a simmer, cook until the lentils are tender, about 25 minutes. Remove from the heat, stir in the vinegar and season with salt and pepper.
While the soup cooks, prepare the cauliflower.

On a baking sheet, spread the cauliflower in an even layer. Drizzle with oil, season with salt, and roast until starting to brown, about 20 minutes. Slice the roll in half and drizzle with oil. About 10 minutes before the cauliflower is done, put the roll, cut-side up, on the baking sheet next to the cauliflower and toast until browned, about 10 minutes.

Ladle the soup into ovenproof bowls and divide the roasted cauliflower evenly between each bowl. Sprinkle the cheese on top. Transfer the bowls to a baking sheet and bake in the hot oven until the cheese melts and is beginning to brown.

Carefully transfer the hot bowl to a plate lined with a napkin or a paper towel before setting on the table and serve the toasted roll on the side.