Preheat the oven to 400°F and line a large baking sheet with parchment paper.

Chop the cauliflower including the core pieces. Transfer to the baking sheet, drizzle with olive oil, and sprinkle with salt and pepper. Toss to coat and spread evenly on the baking sheet. Wrap the shallot and garlic cloves, along with a drizzle of olive oil and a pinch of salt, in a piece of aluminum foil and place it on the baking sheet with the cauliflower. Roast for 30 to 35 minutes, or until the cauliflower is browned around the edges.

In a large pot or Dutch oven, bring the vegetable broth to a simmer over medium heat. Add the roasted cauliflower, shallots, peeled garlic, and thyme and simmer, covered, for 15 minutes. Let cool slightly and transfer to a blender. Add the olive oil, miso paste, mustard, and lemon juice. Blend until smooth. Season to taste with 1/4 to 1/2 teaspoon more salt and more lemon juice. Garnish with microgreens, if desired, and serve.