In a large pot, bring the vegetable broth to a low simmer. 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 miso paste, mustard, olive oil, and lemon juice. Blend until smooth. Season to taste with ¼ to ½ teaspoon more salt and more lemon juice. Garnish with microgreens, if desired, and serve.